Starting in the mid-1990s, the U.S. petroleum industry experienced a wave of mergers, acquisitions, and joint ventures, several of them between large oil companies that had previously competed with each other. For example, Exxon, the largest U.S. oil company, acquired Mobil, the second largest, thus forming ExxonMobil. GAO was asked to examine the effects of the mergers on the U.S. petroleum industry since the 1990s. For this period, GAO examined (1) mergers in the U.S. petroleum industry and why they occurred, (2) the extent to which market concentration (the distribution of market shares among competing firms) and other aspects of market structure in the U.S. petroleum industry have changed as a result of mergers, (3) major changes that have occurred in U.S. gasoline marketing, and (4) how mergers and market concentration in the U.S. petroleum industry have affected U.S. gasoline prices at the wholesale level. Commenting on a draft of GAO's report, FTC asserted that the models were flawed and the analyses unreliable. GAO used state-of-the-art econometric models to examine the effects of mergers and market concentration on wholesale gasoline prices. The models used in GAO's analyses were peer reviewed by independent experts. Thus, GAO believes its analyses are sound.