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UNITED STATES SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Form 10-Q

For the quarterly period ended September 30, 2008

or

Commission file number: 1-6686

THE INTERPUBLIC GROUP OF COMPANIES, INC.

(Exact name of registrant as specified in its charter)

1114 Avenue of the Americas, New York, New York 10036

(Address of principal executive offices) (Zip Code)

(212) 704-1200

(Registrant’s telephone number, including area code)

(Former name, former address and former fiscal year, if changed since last report)

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.

Yes x    No ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See definition of “large accelerated filer,” “accelerated filer,” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

Large accelerated filer x    Accelerated filer ¨    Non-accelerated filer (Do not check if a smaller reporting company) ¨

Smaller reporting company ¨

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).

Yes ¨    No x

The number of shares of the registrant’s common stock outstanding as of October 17, 2008 was 476,540,074.

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INDEX

INFORMATION REGARDING FORWARD-LOOKING DISCLOSURE

This quarterly report on Form 10-Q contains forward-looking statements and when used in this discussion and the financial statements, the words “expect(s)”, “will”, “may”, “could”, and similar expressions are intended to identify forward-looking statements. Statements in this report that are not historical facts, including statements about management’s beliefs and expectations, constitute forward-looking statements. These statements are based on current plans, estimates and projections, and are subject to change based on a number of factors, including those outlined under Item 1A, Risk Factors, in our most recent annual report on Form 10-K. Forward-looking statements speak only as of the date they are made, and we undertake no obligation to update publicly any of them in light of new information or future events.

Forward-looking statements involve inherent risks and uncertainties. A number of important factors could cause actual results to differ materially from those contained in any forward-looking statement. Such factors include, but are not limited to, the following:

Investors should carefully consider these factors and the additional risk factors outlined in more detail under Item 1A, Risk Factors, in our 2007 Annual Report on Form 10-K.

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Part I — FINANCIAL INFORMATION

THE INTERPUBLIC GROUP OF COMPANIES, INC. AND SUBSIDIARIES

CONSOLIDATED STATEMENTS OF OPERATIONS

(Amounts in Millions, Except Per Share Amounts)

(Unaudited)

The accompanying notes are an integral part of these unaudited financial statements.

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THE INTERPUBLIC GROUP OF COMPANIES, INC. AND SUBSIDIARIES

CONDENSED CONSOLIDATED BALANCE SHEETS

(Amounts in Millions)

(Unaudited)

The accompanying notes are an integral part of these unaudited financial statements.

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THE INTERPUBLIC GROUP OF COMPANIES, INC. AND SUBSIDIARIES

CONSOLIDATED STATEMENTS OF CASH FLOWS

(Amounts in Millions)

(Unaudited)

The accompanying notes are an integral part of these unaudited financial statements.

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Notes to Consolidated Financial Statements

(Amounts in Millions, Except Per Share Amounts)

(Unaudited)

Note 1:  Basis of Presentation

The unaudited Consolidated Financial Statements have been prepared by The Interpublic Group of Companies, Inc. (together with its subsidiaries, the “Company”, “Interpublic”, “we”, “us” or “our”) in accordance with accounting principles generally accepted in the United States of America and pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC” or the “Commission”) for reporting interim financial information on Form 10-Q. Accordingly, they do not include certain information and disclosures required for complete financial statements. The preparation of financial statements in accordance with accounting principles generally accepted in the United States requires us to make judgments, assumptions, and estimates that affect the amounts reported and disclosed. Actual results could differ from those estimates. These financial results should be read in conjunction with our 2007 Annual Report on Form 10-K.

In the opinion of management, these unaudited Consolidated Financial Statements include all adjustments of a normal and recurring nature necessary for a fair statement of the information for each period contained therein. Certain reclassifications have been made to prior periods to conform to the current period presentation. The consolidated results for interim periods are not necessarily indicative of results for the full year, as historically our consolidated revenue is higher in the second half of the year than in the first half.

Note 2:  Financings and Related Transactions

Interest Rate Swap

In September 2008, we terminated our interest rate swap agreement executed in June 2008 related to $125.0 in notional amount of our 7.25% Senior Unsecured Notes due 2011 (the “7.25% Notes”). We will receive approximately $3.0 in cash in equal semi-annual installments over the life of the 7.25% Notes. Accordingly, a gain of $2.4 will be amortized as a reduction to interest expense over the remaining term of the 7.25% Notes, resulting in an effective interest rate of 7.1% per annum.

Credit Agreement

In July 2008 we entered into a $335.0 Three-Year Credit Agreement (the “2008 Credit Agreement”) with a syndicate of banks. The 2008 Credit Agreement is a revolving facility, under which amounts borrowed by us or any of our subsidiaries designated under the 2008 Credit Agreement may be repaid and reborrowed, subject to an aggregate lending limit of $335.0 or the equivalent in other currencies, and the aggregate available amount of letters of credit outstanding may decrease or increase, subject to a limit on letters of credit of $200.0 or the equivalent in other currencies. The terms of the 2008 Credit Agreement allow us to increase the aggregate lending commitment to a maximum amount of $485.0 if lenders agree to the additional commitments. Our obligations under the Credit Agreement are unsecured. The 2008 Credit Agreement will expire on July 18, 2011.

4.50% Convertible Senior Notes

In March 2008, holders of approximately $191.0 in aggregate principal amount of our 4.50% Convertible Senior Notes due 2023 (the “4.50% Notes”) exercised their put option that required us to repurchase their 4.50% Notes for cash, pursuant to the terms of the 4.50% Notes. The purchase price was 100% of the principal amount, which we paid from existing cash on hand. We can redeem the remaining 4.50% Notes (approximately $9.0 as of September 30, 2008) for cash on or after September 15, 2009.

Note 3:  Fair Value Measurements

Effective January 1, 2008, we adopted Statement of Financial Accounting Standards No. 157, Fair Value Measurements (“SFAS 157”), which defines fair value, establishes a framework for measuring fair value and expands required disclosures about fair value measurements. Under the standard, fair value refers to the price that would be received to sell an asset or

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Notes to Consolidated Financial Statements – (continued)

(Amounts in Millions, Except Per Share Amounts)

(Unaudited)

paid to transfer a liability in an orderly transaction between market participants in the market in which the reporting entity transacts. The standard clarifies the principle that fair value should be based on the assumptions market participants would use when pricing the asset or liability. The impact of adopting SFAS 157 as of January 1, 2008 was not material to our Consolidated Financial Statements.

FSP FAS 157-1, Application of FASB Statement No. 157 to FASB Statement No. 13 and Other Accounting Pronouncements That Address Fair Value Measurements for Purposes of Lease Classification or Measurement under Statement 13, removed leasing transactions accounted for under SFAS No. 13, Accounting for Leases, and related guidance from the scope of SFAS 157. FSP FAS 157-2, Effective Date of FASB Statement No. 157, deferred the effective date of SFAS 157 for the Company in relation to all nonfinancial assets and nonfinancial liabilities to January 1, 2009.

SFAS 157 establishes a fair value hierarchy which requires us to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. We primarily apply the market approach for recurring fair value measurements. The standard describes three levels of inputs that may be used to measure fair value:

As of September 30, 2008, we held $12.5 in par value of asset backed auction rate securities. Since August 2007, auctions have failed due to insufficient bids from buyers, which required us to adjust the securities to a book value of $6.7 during the fourth quarter of 2007. We intend to hold our auction rate securities until we can recover the full principal and have classified the auction rate securities as long-term investments within other assets in our consolidated balance sheet as of September 30, 2008.

In October 2008, the FASB issued FSP FAS 157-3, Determining the Fair Value of a Financial Asset When the Market for That Asset Is Not Active, which addresses the application of SFAS 157 for illiquid financial instruments. FSP FAS 157-3 clarifies that approaches to determining fair value other than the market approach may be appropriate when the market for a financial asset is not active. We believe that an income approach valuation technique to value our auction rate securities is more representative of fair value than a market approach valuation technique. Specifically, we used the discount rate adjustment technique with a discount rate of 10.84% derived from observed rates of return for comparable assets that are traded in the market. Based on our analysis, we have determined that the fair value of the auction rate securities adequately supports its book value.

The following table presents information about our assets and liabilities measured at fair value on a recurring basis as of September 30, 2008 and indicates the fair value hierarchy of the valuation techniques utilized to determine such fair value.

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Notes to Consolidated Financial Statements – (continued)

(Amounts in Millions, Except Per Share Amounts)

(Unaudited)

The following tables present additional information about assets and liabilities measured at fair value on a recurring basis and for which we utilize Level 3 inputs to determine fair value.

Three months ended September 30, 2008

Realized gains (losses) included in net income for foreign currency derivatives and a minority interest put obligation are reported as a component of other expense and interest expense, respectively.

Note 4:  Supplementary Data

Accrued Liabilities

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Notes to Consolidated Financial Statements – (continued)

(Amounts in Millions, Except Per Share Amounts)

(Unaudited)

As part of the restatement set forth in our 2004 Annual Report on Form 10-K filed in September 2005 (the “2004 Restatement”), we recognized liabilities related to vendor discounts and credits where we had a contractual or legal obligation to rebate such amounts to our clients or vendors. For the nine months ended September 30, 2008, we satisfied $1.4 of these liabilities through cash payments and reductions of certain client receivables. Further reductions of these liabilities were a result of favorable settlements with clients, the release of liabilities due to the lapse of the respective statutes of limitations and foreign currency effects. Also, as part of the 2004 Restatement, we recognized liabilities related to internal investigations and international compensation arrangements.

Restructuring and Other Reorganization-Related Charges (Reversals)

Restructuring and other reorganization-related charges of $3.9 and $11.2 for the three and nine months ended September 30, 2008, respectively, primarily relate to the realignment of our global media operations. See Note 11 for a discussion regarding the creation of our new management entity, Mediabrands, in the second quarter of 2008. In addition, the charges for the nine months ended September 30, 2008 relate to the restructuring program announced at Lowe Worldwide (“Lowe”) during the third quarter of 2007. Net charges primarily consist of leasehold amortization and additional severance expense. Payments during the quarter related to the 2007, 2003 and 2001 programs were approximately $3.0. The total liability balance as of September 30, 2008 for our restructuring programs is $15.7, of which $1.9, $6.9 and $6.9 relate to the 2007, 2003 and 2001 programs, respectively.

Other (Expense) Income

Results of operations for the three and nine months ended September 30, 2008 and 2007 include certain items which are either non-recurring or are not directly associated with our revenue producing operations. These items are included in the other income (expense) line in the Consolidated Statements of Operations.

Sales of businesses and investments — Primarily includes realized gains and losses relating to the sales of businesses, cumulative translation adjustment balances from the liquidation of entities, sales of marketable securities and investments in publicly traded and privately held companies in our Rabbi Trusts, and charges related to declines in the values of investments that are deemed to be other than temporary.

Vendor discounts and credit adjustments — We are in the process of settling our liabilities related to vendor discounts and credits primarily established as part of the 2004 Restatement. These adjustments reflect the reversal of certain liabilities as a result of settlements with clients and vendors or where the statute of limitations has lapsed.

Litigation settlement — During May 2008, the SEC concluded their investigation that began in 2002 into our financial reporting practices resulting in a settlement charge of $12.0.

Note 5:  Acquisitions

The majority of our acquisitions include an initial payment at the time of closing and provide for additional contingent purchase price payments over a specified time. Contingent purchase price payments are recorded within the financial

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Notes to Consolidated Financial Statements – (continued)

(Amounts in Millions, Except Per Share Amounts)

(Unaudited)

statements as an increase to goodwill and other intangible assets once the terms and conditions of the contingent acquisition obligations have been met and the consideration is determinable and distributable, or expensed as compensation in our Consolidated Statements of Operations based on the acquisition agreement and the terms and conditions of employment for the former owners of the acquired businesses.

During the nine months ended September 30, 2008, we completed nine acquisitions, of which the most significant were: a) the remaining interests in an entertainment-marketing agency in North America in which we previously held a 40% interest, b) a digital advertising and communications agency in the United Kingdom, c) a marketing services agency in France, d) a 51% interest in a digital marketing agency in North America, and e) an additional 31.1% interest in a full-service advertising agency in the Middle East which increases our total interest in that agency to 51%. Total cash paid for these acquisitions at closing was $100.8, and we have accrued an additional $51.7 for known deferred payments, primarily related to our acquisition in the Middle East.

For companies acquired during the first nine months of 2008, we made estimates of the fair values of the assets and liabilities for consolidation. The purchase price in excess of the estimated fair value of the tangible net assets acquired was allocated to goodwill and identifiable intangible assets. These acquisitions do not have significant amounts of tangible assets, therefore a substantial portion of the total consideration has been allocated to goodwill and identifiable intangible assets (approximately $183.0). The purchase price allocations for our acquisitions are substantially complete, however certain of these allocations are based on estimates and assumptions and are subject to change. The final determination of the estimated fair value of the acquired net assets will be completed as soon as possible, but no later than one year from the acquisition date. All acquisitions during the first nine months of 2008 are included in the IAN operating segment. Pro forma information, as required by SFAS No. 141, Business Combinations, related to these acquisitions is not presented because the impact of these acquisitions, either individually or in the aggregate, on the Company’s consolidated results of operations is not significant.

During the nine months ended September 30, 2007, we completed six acquisitions, of which the most significant were: a) a full-service advertising agency in Latin America, b) Reprise Media, which is a full-service search engine marketing firm in North America, and c) the remaining interests in two full-service advertising agencies in India in which we previously held 49% and 51% interests. Total cash paid during the first nine months of 2007 for these acquisitions was $112.8.

Details of cash paid for current and prior years’ acquisitions are as follows:

In addition, for the nine months ended September 30, 2007, we acquired $8.1 of marketable securities held by one of our acquisitions.

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Notes to Consolidated Financial Statements – (continued)

(Amounts in Millions, Except Per Share Amounts)

(Unaudited)

Note 6:  Employee Benefits

We have a defined benefit plan which covers substantially all regular U.S. employees employed through March 31, 1998. In addition, some of our agencies have additional domestic plans. These plans are closed to new participants. We also have numerous plans outside of the U.S., some of which are funded, while others provide payments at the time of retirement or termination under applicable labor laws or agreements. Some of our domestic and foreign subsidiaries also provide postretirement health benefits to eligible employees and their dependents. Additionally, some of our domestic subsidiaries provide postretirement life insurance to eligible employees. Certain immaterial foreign pension plans have been excluded from the below tables. The components of net periodic cost for the domestic pension plans, the principal foreign pension plans and the postretirement benefit plans are as follows:

During the three and nine months ended September 30, 2008, we made contributions of $5.7 and $21.9, respectively, to our foreign pension plans. For the remainder of 2008, we expect to contribute $6.2 to our foreign pension plans. In accordance with changes required by the Pension Protection Act of 2006, we contributed $2.3 to our domestic pension plans during the three and nine months ended September 30, 2008. For the remainder of 2008, we do not expect to make any additional contributions to our domestic pension plans.

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Notes to Consolidated Financial Statements – (continued)

(Amounts in Millions, Except Per Share Amounts)

(Unaudited)

Note 7:  Stock-Based Compensation

During the nine months ended September 30, 2008 we granted the following stock-based compensation awards under our 2006 Performance Incentive Plan:

Stock options are granted with the exercise price equal to the fair market value of our common stock on the grant date, are generally exercisable between two and four years from the grant date and expire ten years from the grant date (or earlier in the case of certain terminations of employment).

Stock-settled awards include restricted stock and restricted stock units (“RSUs”) expected to be settled with our common stock and generally vest over three years. RSUs that are expected to be settled in stock and restricted stock are amortized over the vesting period based on the grant date fair value of awards.

Cash-settled awards include RSUs expected to be settled in cash and generally vest over three years. We adjust our fair value measurement for RSUs that are expected to be settled in cash quarterly based on our share price, and we amortize stock-based compensation expense over the vesting periods based upon the quarterly-adjusted fair value.

Performance-based awards are a form of stock-based compensation in which the number of shares or units ultimately received by the participant depends on Company and/or individual performance against specific performance targets and can be settled in cash or shares. The awards generally vest over a three-year period subject to the participant’s continuing employment and the achievement of certain performance objectives. We amortize stock-based compensation expense for the estimated number of performance-based awards that we expect to vest over the vesting period using the grant-date fair value of the shares, except for the cash-settled performance-based-awards, which we amortize using the quarterly-adjusted fair value.

Note 8:  Comprehensive (Loss) Income

Note 9:  Earnings (Loss) Per Share

Earnings (loss) per basic common share equals net income (loss) applicable to common stockholders divided by the weighted average number of common shares outstanding for the applicable period. Diluted earnings (loss) per share equals

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Notes to Consolidated Financial Statements – (continued)

(Amounts in Millions, Except Per Share Amounts)

(Unaudited)

net income (loss) applicable to common stockholders adjusted to exclude, if dilutive, preferred stock dividends, allocation to participating securities and interest expense related to potentially dilutive securities divided by the weighted average number of common shares outstanding, plus any additional common shares that would have been outstanding if potentially dilutive shares had been issued. The following sets forth basic and diluted earnings (loss) per common share applicable to common stock.

Basic and diluted shares outstanding and loss per share are equal for the three and nine months ended September 30, 2007 because our potentially dilutive securities are antidilutive as a result of the net loss applicable to common stockholders in each period.

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Notes to Consolidated Financial Statements – (continued)

(Amounts in Millions, Except Per Share Amounts)

(Unaudited)

The following table presents the potential shares excluded from diluted earnings (loss) per share because the effect of including these potential shares would be antidilutive.

Note 10:  Taxes

The effective tax rates for the three and nine months ended September 30, 2008 were 41.6% and 52.2%, respectively. For the three and nine months ended September 30, 2008, the differences between the effective tax rates and the statutory rate of 35% is primarily due to state and local taxes, losses in certain foreign locations where we receive no tax benefit due to 100% valuation allowances and, for nine months ended September 30, 2008, the SEC settlement provision for which we receive no tax benefit and the release of valuation allowances in jurisdictions where we believe it is more likely than not that we will realize our deferred tax assets. The tax provision for the first nine months of 2007 was favorably impacted by net reversals of tax reserves, primarily related to previously unrecognized tax benefits related to various items of income and expense, including approximately $80.0 for certain worthless securities deductions associated with investments in consolidated subsidiaries, which was a result of the completion of a tax examination.

With respect to all tax years open to examination by U.S. Federal and various state, local, and non-U.S. tax authorities, we currently anticipate that the total unrecognized tax benefits will decrease by an amount between $35.0 and $45.0 in the next twelve months, a portion of which will affect the effective tax rate, primarily as a result of the settlement of tax examinations and the lapsing of statutes of limitation. This net decrease is related to various items of income and expense, including transfer pricing adjustments and adjustments related to the 2004 Restatement. For this purpose, we expect to complete our discussions with the IRS appeals division regarding the years 1997 through 2004 within the next twelve months.

In December 2007, the IRS commenced its examination for the 2005 and 2006 tax years. In addition, we have various tax years under examination by tax authorities in various countries, such as the United Kingdom, and in various states, such as New York, in which we have significant business operations. It is not yet known whether these examinations will, in the aggregate, result in our paying additional taxes. We believe our tax reserves are adequate in relation to the potential for additional assessments in each of the jurisdictions in which we are subject to taxation. We regularly assess the likelihood of additional tax assessments in those jurisdictions and, if necessary, adjust our reserves as additional information or events require.

With limited exceptions, we are no longer subject to U.S. income tax audits for the years prior to 1997, state and local income tax audits for the years prior to 1999, or non-U.S. income tax audits for the years prior to 2000.

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Notes to Consolidated Financial Statements – (continued)

(Amounts in Millions, Except Per Share Amounts)

(Unaudited)

On October 3, 2008, the Emergency Economic Stabilization Act of 2008 (the “Act”) was enacted in the United States. The Act primarily contains provisions which address the U.S. banking and financial industry bailout. In addition, it contains various tax provisions. However, based on our review we do not anticipate any material impact to our financial position from the tax provisions contained in the Act.

Note 11:  Segment Information

On July 9, 2008 we announced the creation of a management entity called Mediabrands to oversee our media assets that are included in our Integrated Agency Networks (“IAN”) segment. The new entity provides oversight of operational efficiency and increased collaboration across our media units. Our global media networks, Initiative and Universal McCann, continue to operate as independent entities, each aligned where appropriate with its respective full-service marketing network partner. The previously existing entities that comprise Mediabrands remain in the IAN segment.

We have two reportable segments: IAN, which is comprised of Draftfcb, Lowe, McCann Worldgroup and Mediabrands, and Constituency Management Group (“CMG”), which is comprised of the bulk of our specialist marketing service offerings. We also report results for the Corporate and other group. The profitability measure employed by our chief operating decision maker for allocating resources to our operating divisions and assessing operating division performance is operating income (loss), excluding the impact of restructuring and other reorganization-related charges (reversals) and long-lived asset impairment and other charges. Segment information is presented consistently with the basis described in our 2007 Annual Report on Form 10-K. Summarized financial information concerning our reportable segments is shown in the following table.