What is the proportionate punishment for conduct that is neither harmful nor wrongful? A likely response to that is that one ought not to be punished at all for such conduct. It is, however, common for the state to punish harmless conduct the wrongfulness of which is not always apparent. Take, for example, the requirement that those who give investment advice for compensation do so only after registering as an investment advisor. Advising a person on how to invest his or (...) her funds and accepting a fee for the advice without registering with the government does not seem harmful or wrongful, so long as no fraud is involved, the relevant parties understand the relevant risks, and so on. But practicing investment advising without registering is a crime for which one may be convicted and punished. When one thinks of crimes, paradigmatic offenses are crimes like murder, rape, and robbery, but offenses like failure to register as an investment advisor are different. But in what way? One standard explanation is the distinction between two types of offenses, malum in se and malum prohibitum. Some offenses, like murder, are wrongs “in themselves” whereas other offenses, like investment advising without registering as an advisor, are wrongs because they have been prohibited. The question this Essay asks is how we should think about proportionality of punishment when punishing such mala prohibita offenses. This Essay presents a framework for such proportionality determinations and raises some challenges such a framework would need to confront. (shrink)