Nigel Walker’s first principle of criminalization declares that ‘Prohibitions should not be included in the criminal law for the sole purpose of ensuring that breaches of them are visited with retributive punishment’. I argue that we should reject this principle, for ‘mala prohibita’ as well as for ‘mala in se’: conduct should be criminalized in order to ensure (as far as we reasonably can) that those who engage in it receive retributive punishment. In the course of the argument, I show why we should not see the criminal law as consisting in ‘prohibitions’; I explain different species of mala prohibita, and show how their commission does involve genuine wrongdoing; and I show the importance of distinguishing the question of regulation from that of sanction.