The fundamental requirement of Anglo-American criminal law is that crime must consist of the concurrence of a guilty mind—a mens rea—with a guilty act—an actus reus. And yet, the criminal law is shot through with discordant lumps of strict liability—crimes for which no mens rea is required. Ignoring the conventional normative objections to this aberration, I distinguish two different types of strict criminal liability: the type that arose at common law and the type associated with the public welfare offenses that are the product of twentieth and twenty-first century legislation. Using famous cases as exemplars, I analyze the two types of strict liability, and then examine the purposes served and incentives created by subjecting individuals to strict liability. I conclude that common law strict liability is rational in that it advances the purposes of the criminal law, while the public welfare offenses are at best pointless and at worst counterproductive. I suggest that in this respect the common law contains more wisdom than the results of the legislative process.