This Article argues that contracts that violate Section 1 of the Sherman Act should not be enforceable. Although seemingly modest, courts do not accept this proposition. When a defendant in a breach of contract action raises the defense of \u201cillegality\u201d under the Sherman Act, courts will likely reject the defense unless the contractual provision at issue is unlawful \u201con its face.\u201d The practical consequence of this rule is that the antitrust illegality defense will likely fail in most litigated breach of contract cases. Parties only rarely attempt to enforce obvious violations of Section 1 of the Sherman Act, like price fixing or other per se unlawful agreements, in court. However, parties do attempt to enforce agreements that are potentially unlawful under antitrust\u2019s \u201crule of reason.\u201d These sorts of agreements are not typically unlawful on their face, like agreements to fix prices, but may be found unlawful after an examination of the markets at issue. The resources required to conduct such a market examination, however, can be significant. As a result, courts have been reluctant to allow defendants to transform \u201csimple\u201d breach of contract actions into complex antitrust cases and will accordingly reject the defense that a contract is unenforceable because it violates the Sherman Act under the rule of reason. This practice is wrong. It reflects not only bad policy, but also misapplications of federal common law and preemption law. No academic commentator has published in any depth on this topic. But it is a topic that should be engaged because locking parties into anticompetitive contracts is a significant problem in today\u2019s economy. This Article attempts to provide a rigorous analysis of the origins and implications of today\u2019s problematic doctrine. It argues that agreements that violate Section 1 of the Sherman Act, whether under the per se rule or rule of reason, should not be enforceable. It proposes a new framework for assessing the defense of antitrust illegality specifically but also assessing the defense that a contract violates a federal law generally.<\/p>\n\n\n\n