A Practice Note providing an overview of Section 1 of the Sherman Act (15 U.S.C. § 1), the federal antitrust law prohibiting unreasonable restraints of trade. The Note discusses the elements to establish an unreasonable agreement under Section 1, the rule of reason, the per se rule, examples of horizontal and vertical agreements that violate Section 1, enforcement by the Antitrust Division of the Department of Justice, the Federal Trade Commission, state attorneys general, and private parties, as well as defenses to and exemptions from Section 1 claims.