United States antitrust authorities have brought a number of enforcement actions in recent years challenging conduct of firms proposing mergers, acquisitions, and joint ventures. These cases have attacked covenants restricting activities pending closing of proposed transactions as well as initial efforts to integrate operations and even exchanges of information among parties pro to merge. The government is proceeding on theories that such conduct is illegal "gun-jumping" or "price-fixing," which is unlawful under the Hart-Scott-Rodino Act, the Sherman Act, or the Federal Trade Commission Act. This Article outlines the legal framework for analyzing pre-closing conduct by parties proposing a transaction, describes the conduct that has led firms into trouble in recent enforcement actions, and provides practical guidelines as to what is lawful and unlawful in this confusing area of the law.