UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One): x Quarterly report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934. For the quarterly period ended March 31, 2019. ¨ Transition report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934. Commission File Number: 001-14195 AMERICAN TOWER CORPORATION (Exact name of registrant as specified in its charter) Delaware 65-0723837 (State or other jurisdiction of Incorporation or Org ...

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K (Mark One): ý Annual report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934. For the fiscal year ended December 31, 2018 ¨ Transition report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934. For the transition period from to Commission File Number: 001-14195 American Tower Corporation (Exact name of registrant as specified in its charter) Delaware 65-0723837 (State ...