Many legal rules turn on a party's state of mind - or intent - with respect to some action or consequence. Legal scholars have long debated the contours of such requirements and the sorts of proof required for them. Intent has been an especially controversial issue in antitrust law. This paper provides a theory of legal standards that explains the role of intent analysis in antitrust and in other areas of the law. The authors argue that intent requirements, and many other legal rules, can be understood by focusing on the goal of minimizing the expected costs from legal errors. After developing a positive theory of intent standards, they apply the theory to antitrust to show that it explains both the allocation of and proof requirements for the specific intent standards in antitrust doctrine. They then use the Microsoft case as a concrete study of the function of intent rules in antitrust.