The article discusses a theory of vagueness avoidance as a tool for constraining penal statutes in the U.S. It explains the goals of the Supreme Court in engaging in vagueness avoidance, including promoting the legality principle in criminal law by preventing retroactive crime definition through judicial innovation. It explores the canons of constitutional avoidance, the domain and limits of vagueness avoidance, as well as cases of explicit vagueness avoidance.