(Mark One):

Commission File Number: 001-14195

(Exact name of registrant as specified in its charter)

116 Huntington Avenue

Boston, Massachusetts 02116

(Address of principal executive offices)

Telephone Number (617) 375-7500

(Registrant's telephone number, including area code)

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 has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    Yes   x     No   ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, smaller reporting company or an emerging growth company. See the definitions of "large accelerated filer," "accelerated filer," "smaller reporting company," and "emerging growth company" in Rule 12b-2 of the Exchange Act. (Check One):

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.    ¨

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act):    Yes   ¨     No   x

As of April 20, 2017 , there were 425,009,451 shares of common stock outstanding.

AMERICAN TOWER CORPORATION

TABLE OF CONTENTS

QUARTERLY REPORT ON FORM 10-Q

FOR THE QUARTER ENDED MARCH 31, 2017

AMERICAN TOWER CORPORATION AND SUBSIDIARIES

CONSOLIDATED BALANCE SHEETS

See accompanying notes to unaudited consolidated and condensed consolidated financial statements.

AMERICAN TOWER CORPORATION AND SUBSIDIARIES

CONSOLIDATED STATEMENTS OF OPERATIONS

(in thousands, except per share data)

See accompanying notes to unaudited consolidated and condensed consolidated financial statements.

AMERICAN TOWER CORPORATION AND SUBSIDIARIES

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS)

(in thousands)

See accompanying notes to unaudited consolidated and condensed consolidated financial statements.

AMERICAN TOWER CORPORATION AND SUBSIDIARIES

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS

See accompanying notes to unaudited consolidated and condensed consolidated financial statements.

AMERICAN TOWER CORPORATION AND SUBSIDIARIES

CONSOLIDATED STATEMENTS OF EQUITY

(in thousands, except share data)

See accompanying notes to unaudited consolidated and condensed consolidated financial statements.

American Tower Corporation (together with its subsidiaries, "ATC" or the "Company") is one of the largest global real estate investment trusts and a leading independent owner, operator and developer of multitenant communications real estate. The Company's primary business is the leasing of space on communications sites to wireless service providers, radio and television broadcast companies, wireless data providers, government agencies and municipalities and tenants in a number of other industries, which the Company refers to as its property operations. Additionally, the Company offers tower-related services in the United States, including site acquisition, zoning and permitting and structural analysis, which primarily support its site leasing business, including the addition of new tenants and equipment on its sites, which the Company refers to as its services operations.

The Company's portfolio primarily consists of towers it owns and towers it operates pursuant to long-term lease arrangements, as well as distributed antenna system ("DAS") networks, which provide seamless coverage solutions in certain in-building and certain outdoor wireless environments. In addition to the communications sites in its portfolio, the Company manages rooftop and tower sites for property owners under various contractual arrangements. The Company also holds other telecommunications infrastructure and property interests that it leases to communications service providers and third-party tower operators.

ATC is a holding company that conducts its operations through its directly and indirectly owned subsidiaries and its joint ventures. ATC's principal domestic operating subsidiaries are American Towers LLC and SpectraSite Communications, LLC. ATC conducts its international operations primarily through its subsidiary, American Tower International, Inc., which in turn conducts operations through its various international holding and operating subsidiaries and joint ventures.

The Company operates as a real estate investment trust for U.S. federal income tax purposes ("REIT"). Accordingly, the Company generally is not subject to U.S. federal income taxes on income generated by its REIT operations, including the income derived from leasing space on its towers, as the Company receives a dividends paid deduction for distributions to stockholders that generally offsets its REIT income and gains. However, the Company remains obligated to pay U.S. federal income taxes on earnings from its domestic taxable REIT subsidiaries ("TRSs"). In addition, the Company's international assets and operations, regardless of their designation for U.S. tax purposes, continue to be subject to taxation in the foreign jurisdictions where those assets are held or those operations are conducted.

The use of TRSs enables the Company to continue to engage in certain businesses while complying with REIT qualification requirements. The Company may, from time to time, change the election of previously designated TRSs to be included as part of the REIT. As of March 31, 2017 , the Company's REIT-qualified businesses included its U.S. tower leasing business, most of its operations in Costa Rica, Germany and Mexico and a majority of its services segment and indoor DAS networks business.

The accompanying consolidated and condensed consolidated financial statements have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission. The financial information included herein is unaudited. However, the Company believes that all adjustments considered necessary for a fair presentation of its financial position and results of operations for such periods have been included herein. The consolidated and condensed consolidated financial statements and related notes should be read in conjunction with the Company's Annual Report on Form 10-K for the year ended December 31, 2016 (the " 2016 Form 10-K"). The results of operations for the three months ended March 31, 2017 are not necessarily indicative of the results that may be expected for the entire year.

Principles of Consolidation and Basis of Presentation -The accompanying consolidated and condensed consolidated financial statements include the accounts of the Company and those entities in which it has a controlling interest. Investments in entities that the Company does not control are accounted for using the equity or cost method, depending upon the Company's ability to exercise significant influence over operating and financial policies. All intercompany accounts and transactions have been eliminated. As of March 31, 2017 , the Company has a controlling interest in two joint ventures, one in Ghana and one in Uganda, with MTN Group Limited ("MTN Group"). The joint ventures are controlled by holding companies of which a wholly owned subsidiary of the Company holds a 51% controlling interest and a wholly owned subsidiary of MTN Group holds a 49% noncontrolling interest. The Company holds a 51%

controlling interest, and PGGM holds a 49% noncontrolling interest, in a joint venture ("ATC Europe") in Europe. In addition, the Company holds an approximate 75% controlling interest, and the South African investors hold an approximate 25% noncontrolling interest, in a subsidiary of the Company in South Africa. The Company also holds a 51% controlling interest in ATC Telecom Infrastructure Private Limited ("ATC TIPL"), formerly Viom Networks Limited ("Viom") in India.

Significant Accounting Policies -The Company's significant accounting policies are described in note 1 to the Company's consolidated financial statements included in the 2016 Form 10-K. There have been no material changes to the Company's significant accounting policies during the three months ended March 31, 2017 .

Accounting Standards Updates -In May 2014, the Financial Accounting Standards Board (the "FASB") issued new revenue recognition guidance, which requires an entity to recognize revenue in an amount that reflects the consideration to which the entity expects to be entitled in exchange for the transfer of promised goods or services to customers. The standard will replace most existing revenue recognition guidance and will become effective for the Company on January 1, 2018. Early adoption is permitted for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2016. The standard permits the use of either the retrospective or cumulative effect transition method. Leases are not included in the scope of this standard. The revenue to which the Company must apply this standard is generally limited to services revenue, certain power and fuel charges and other fees charged to customers. As of March 31, 2017, this revenue was approximately 13% of total revenue. Although the Company is still assessing the impact of this standard on its financial statements, it does not expect changes in the timing of revenue recognition to be material to its financial statements.

In January 2016, the FASB issued new guidance on the recognition and measurement of financial assets and financial liabilities. The guidance amends certain aspects of recognition, measurement, presentation and disclosure of financial instruments. This standard is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2017. The Company does not expect the adoption of this guidance to have a material effect on its financial statements.

In February 2016, the FASB issued new guidance on the accounting for leases. The guidance amends the existing accounting standards for lease accounting, including the requirement that lessees recognize right of use assets and lease liabilities for leases with terms greater than twelve months in the statement of financial position. Under the new guidance, lessor accounting is largely unchanged. This guidance is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2018. The standard is required to be applied using a modified retrospective approach for all leases existing at, or entered into after, the beginning of the earliest comparative period presented. The Company (i) has established a multidisciplinary team to assess and implement the new guidance, (ii) expects the guidance to have a material impact on its consolidated balance sheets due to the recording of right of use assets and lease liabilities for leases in which it is a lessee and which it is currently treating as operating leases and (iii) continues to evaluate the impact of the new guidance.

In November 2016, the FASB issued new guidance on amounts described as restricted cash or restricted cash equivalents within the statement of cash flows. The guidance requires amounts generally described as restricted cash and restricted cash equivalents be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period balances on the statement of cash flows. The guidance is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2017. The standard is required to be applied using a retrospective transition method to each period presented. The Company does not expect the adoption of this guidance to have a material effect on its financial statements.

In January 2017, the FASB issued new guidance that clarifies the definition of a business that an entity uses to determine whether a transaction should be accounted for as an asset acquisition (or disposal) or a business combination. The Company early adopted this guidance during the quarter ended March 31, 2017. As a result, the Company expects that more transactions will be accounted for as asset acquisitions instead of business combinations.

In January 2017, the FASB issued new guidance that simplifies the accounting for goodwill impairments by eliminating Step 2 from the goodwill impairment test. The guidance requires, among other things, recognition of an impairment loss when the carrying value of a reporting unit exceeds its fair value. The loss recognized is limited to the total amount of

goodwill allocated to that reporting unit. The guidance is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The Company does not expect the adoption of this guidance to have a material effect on its financial statements.

2.    PREPAID AND OTHER CURRENT ASSETS

Prepaid and other current assets consisted of the following (in thousands):

3.    GOODWILL AND OTHER INTANGIBLE ASSETS

The changes in the carrying value of goodwill for the Company's business segments were as follows (in thousands):

_______________

(1)    Balances have been revised to reflect purchase accounting measurement period adjustments.

The Company's other intangible assets subject to amortization consisted of the following:

_______________

The acquired network location intangibles represent the value to the Company of the incremental revenue growth that could potentially be obtained from leasing the excess capacity on acquired communications sites. The acquired tenant-related intangibles typically represent the value to the Company of tenant contracts and relationships in place at the time of an acquisition or similar transaction, including assumptions regarding estimated renewals.

The Company amortizes its acquired network location intangibles and tenant-related intangibles on a straight-line basis over their estimated useful lives. As of March 31, 2017 , the remaining weighted average amortization period of the Company's intangible assets, excluding the TV Azteca Economic Rights detailed in note 5 to the Company's consolidated financial statements included in the 2016 Form 10-K, was 16 years. Amortization of intangible assets for the three months ended March 31, 2017 and 2016 was $183.2 million and $151.8 million , respectively. Based on current exchange rates, the Company expects to record amortization expense as follows over the remaining current year and the five subsequent years (in millions):

4.    ACCRUED EXPENSES

Accrued expenses consisted of the following (in thousands):

5 .    LONG-TERM OBLIGATIONS

Outstanding amounts under the Company's long-term obligations, reflecting discounts, premiums, debt issuance costs and fair value adjustments due to interest rate swaps consisted of the following (in thousands):

_______________

Current portion of long-term obligations- The Company's current portion of long-term obligations primarily includes (i) $999.0 million under the 4.500% senior unsecured notes due 2018, (ii) $498.9 million under the Secured Tower Revenue Securities, Series 2013-1A and (iii) 9.1 billion INR ( $140.4 million ) of India indebtedness.

Bank Facilities

2013 Credit Facility- During the three months ended March 31, 2017 , the Company borrowed an aggregate of $1.8 billion and repaid an aggregate of $517.9 million of revolving indebtedness under its multicurrency senior unsecured revolving credit facility entered into in June 2013, as amended (the "2013 Credit Facility"). The Company used the borrowings to fund acquisitions, repay existing indebtedness and for general corporate purposes.

2014 Credit Facility- During the three months ended March 31, 2017 , the Company borrowed an aggregate of $200.0 million and repaid an aggregate of $405.0 million of revolving indebtedness under its senior unsecured revolving credit facility entered into in January 2012 and amended and restated in September 2014, as further amended (the "2014 Credit Facility").

As of March 31, 2017 , the key terms under the 2013 Credit Facility, the 2014 Credit Facility and the Company's unsecured term loan entered into in October 2013, as amended (the "Term Loan") were as follows:

_______________

(1)    LIBOR means the London Interbank Offered Rate.

(2)    Fee on undrawn portion of each credit facility.

(3)    Subject to two optional renewal periods.

Repayment of 2012 GTP Notes and Unison Notes and Redemption of Senior Notes -On February 15, 2017, the Company repaid the $173.5 million remaining principal amount outstanding under the Secured Cellular Site Revenue Notes, Series 2012-2 Class A, Series 2012-2 Class B and Series 2012-2 Class C issued by GTP Cellular Sites, LLC, plus prepayment consideration and accrued and unpaid interest. The Company recorded a loss on retirement of long-term obligations of $1.8 million , which includes prepayment consideration of $7.2 million offset by the remaining portion of the unamortized premium.

On February 15, 2017, the Company repaid the $129.0 million principal amount outstanding under the Secured Cellular Site Revenue Notes, Series 2010-2, Class C and Series 2010-2, Class F issued by Unison Ground Lease Funding, LLC, plus prepayment consideration and accrued and unpaid interest. The Company recorded a loss on retirement of long-term obligations of $14.5 million , which includes prepayment consideration of $18.3 million offset by the remaining portion of the unamortized premium.

On February 10, 2017, the Company redeemed all of the outstanding 7.25% senior unsecured notes due 2019 (the " 7.25% Notes") at a price equal to 112.0854% of the principal amount, plus accrued and unpaid interest up to, but excluding, February 10, 2017, for an aggregate redemption price of $341.4 million , including $5.1 million in accrued and

unpaid interest. The Company recorded a loss on retirement of long-term obligations of $39.2 million , which includes prepayment consideration of $ 36.3 million and the remaining portion of the unamortized discount and deferred financing costs. Upon completion of the redemption, none of the 7.25% Notes remained outstanding.

The repayments and the redemption were funded with borrowings under the 2013 Credit Facility and cash on hand.

6.    FAIR VALUE MEASUREMENTS

The Company determines the fair value of its financial instruments based on the fair value hierarchy, which requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. Below are the three levels of inputs that may be used to measure fair value:

Items Measured at Fair Value on a Recurring Basis -The fair values of the Company's financial assets and liabilities that are required to be measured on a recurring basis at fair value were as follows (in thousands):

_______________

During the three months ended March 31, 2017 , the Company has made no changes to the methods described in note 11 to the Company's consolidated financial statements in the 2016 Form 10-K that it used to measure the fair value of its interest rate swap agreements, the embedded derivative in one of its lease agreements and acquisition-related contingent consideration. The changes in fair value during the three months ended March 31, 2017 and 2016 were not material to the consolidated financial statements. As of March 31, 2017 , the Company estimated the value of all potential acquisition-related contingent consideration payments to be between zero and $48.4 million .

Items Measured at Fair Value on a Nonrecurring Basis

Assets Held and Used -The Company's long-lived assets are recorded at amortized cost and, if impaired, are adjusted to fair value using Level 3 inputs. During the three months ended March 31, 2017 and 2016, the Company did not record any material asset impairment charges. There were no other items measured at fair value on a nonrecurring basis during the three months ended March 31, 2017 or 2016.

Fair Value of Financial Instruments -The Company's financial instruments for which the carrying value reasonably approximates fair value at March 31, 2017 and December 31, 2016 include cash and cash equivalents, restricted cash, accounts receivable and accounts payable. The Company's estimates of fair value of its long-term obligations, including the current portion, are based primarily upon reported market values. For long-term debt not actively traded, fair value is estimated using either indicative price quotes or a discounted cash flow analysis using rates for debt with similar terms and maturities. As of March 31, 2017 and December 31, 2016 , the carrying value of long-term obligations, including the current portion, was $18.9 billion and $18.5 billion , respectively. As of March 31, 2017 , the fair value of long-term obligations, including the current portion, was $19.3 billion , of which $11.6 billion was measured using Level 1 inputs and $7.7 billion was measured using Level 2 inputs. As of December 31, 2016 , the fair value of long-term obligations, including the current portion, was $18.8 billion , of which $11.8 billion was measured using Level 1 inputs and $7.0 billion was measured using Level 2 inputs.

7.    INCOME TAXES

The Company provides for income taxes at the end of each interim period based on the estimated effective tax rate ("ETR") for the full fiscal year. Cumulative adjustments to the Company's estimate are recorded in the interim period in which a change in the estimated annual ETR is determined. Under the provisions of the Internal Revenue Code of 1986, as amended, the Company may deduct amounts distributed to stockholders against the income generated by its REIT operations. The Company continues to be subject to income taxes on the income of its TRSs and income taxes in foreign jurisdictions where it conducts operations. In addition, the Company is able to offset certain income by utilizing its net operating losses, subject to specified limitations.

The Company provides valuation allowances if, based on the available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. Management assesses the available positive and negative evidence to estimate if sufficient future taxable income will be generated to use the existing deferred tax assets.

As of March 31, 2017 and December 31, 2016 , the total unrecognized tax benefits that would impact the ETR, if recognized, were approximately $106.7 million and $102.9 million , respectively. The amount of unrecognized tax benefits during the three months ended March 31, 2017 includes additions to the Company's existing tax positions of $1.9 million and foreign currency fluctuations of $3.5 million . Unrecognized tax benefits are expected to change over the next 12 months if certain tax matters ultimately settle with the applicable taxing jurisdiction during this time frame, as described in note 12 to the Company's consolidated financial statements included in the 2016 Form 10-K. The impact of the amount of these changes to previously recorded uncertain tax positions could range from zero to $11.2 million .

The Company recorded penalties and income tax-related interest expense during the three months ended March 31, 2017 and 2016 of $1.3 million and $3.2 million , respectively. As of March 31, 2017 and December 31, 2016 , the total amount of accrued income tax related interest and penalties included in the consolidated balance sheets was $27.0 million and $24.3 million , respectively.

8.    STOCK-BASED COMPENSATION

Summary of Stock-Based Compensation Plans -The Company maintains equity incentive plans that provide for the grant of stock-based awards to its directors, officers and employees. The 2007 Equity Incentive Plan, as amended (the "2007 Plan") provides for the grant of non-qualified and incentive stock options, as well as restricted stock units, restricted stock and other stock-based awards. Exercise prices for non-qualified and incentive stock options are not less than the fair value of the underlying common stock on the date of grant. Equity awards typically vest ratably, generally over four years for time-based restricted stock units ("RSUs") and stock options and three years for performance-based restricted stock units ("PSUs"). Stock options generally expire ten years from the date of grant. As of March 31, 2017 , the Company had the ability to grant stock-based awards with respect to an aggregate of 8.4 million shares of common stock under the 2007 Plan. In addition, the Company maintains an employee stock purchase plan ("ESPP") pursuant to which eligible employees may purchase shares of the Company's common stock on the last day of each bi-annual offering period at a  15%  discount of the lower of the closing market value on the first or last day of such offering period. The offering periods run from June 1 through November 30 and from December 1 through May 31 of each year.

During the three months ended March 31, 2017 and 2016 , the Company recorded and capitalized the following stock-based compensation expense (in thousands):