During the late 1990s, many petroleum companies merged to stay profitable while crude oil prices were low, and in recent years mergers have continued. Congress and others have concerns about the impact mergers might be having on competition in U.S. petroleum markets. The FTC has the authority to maintain competition in the petroleum industry (PI) and reviews proposed mergers to determine whether they are likely to diminish competition or increase prices, among other things. This report examines: (1) mergers in the U.S. PI and changes in market concentration since 2000; and (2) the steps FTC uses to maintain competition in the U.S. PI, and the roles other fed. and state agencies play in monitoring PI markets. Charts and tables.