A Practice Note providing an overview of how the US federal antitrust agencies analyze mergers, acquisitions, and joint ventures. This Note discusses the US Department of Justice (DOJ) and Federal Trade Commission (FTC) 2023 Merger Guidelines, including the frameworks used to determine whether a merger might substantially lessen competition or tend to create a monopoly. It also discusses the agencies' approach to defining the relevant product and geographic markets, measuring market concentration under the Herfindahl-Hirschman Index (HHI), potential competition, platform markets, serial acquisitions, partial ownership. unilateral and coordinated competitive effects, entrenching dominance, entry, efficiencies, and the failing firm defense.