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UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 FORM 10 - K Annual Report Pursuant to Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 For the fiscal year ended December 31 , 2020 Transition Report Pursuant to Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 For the transition period from Commission file number 1-09761 ARTHUR J. GALLAGHER & CO . ( Exact name of registrant as specified in its charter ) DELAWARE ( State or other jurisdiction of incorporation or organization ) 2850 Golf Road Rolling Meadows , Illinois ( Address of principal executive offices ) to Title of each class Common Stock , par value $ 1.00 per share 36-2151613 ( I.R.S. Employer Identification Number ) 60008-4050 ( Zip Code ) Registrant's telephone number , including area code ( 630 ) 773-3800 Securities registered pursuant to Section 12 ( b ) of the Act : Trading Symbol ( s ) AJG Securities registered pursuant to Section 12 ( g ) of the Act : None Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Act . 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 + No □ . Yes > No. Yes No . Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted 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 such files ) . Yes > No □ . Name of each exchange on which registered New York Stock Exchange Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company , or emerging growth company . See definitions of " large accelerated filer , " " accelerated filer , " " smaller reporting company , " and " emerging growth company " in Rule 12b - 2 of the Exchange Act . Large accelerated filer Non - accelerated filer Accelerated filer Smaller reporting company Emerging growth company 0 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 has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C. 726 ( b ) ) by the registered public accounting firm that prepared or issued its audit report Yes > No. Indicate by check mark whether the registrant is a shell company ( as defined in Rule 12b - 2 of the Exchange Act ) . Yes No . The aggregate market value of the voting common equity held by non - affiliates of the registrant , computed by reference to the last reported price at which the registrant's common equity was sold on June 30 , 2020 ( the last day of the registrant's most recently completed second quarter ) was $ 16,249,616,000 . The number of outstanding shares of the registrant's Common Stock , $ 1.00 par value , as of January 31 , 2021 was 193,740,000 . Documents incorporated by reference : Portions of Arthur J. Gallagher & Co.'s definitive 2021 Proxy Statement are incorporated by reference into this Form 10 - K in response to Part III to the extent described herein .