If conduct must be wrongful in order to be justifiably criminalised, how should its wrongfulness be established? I examine a conception of wrongfulness put forward by A. P. Simester, which makes wrongfulness turn on whether the reasons favouring the performance of an action are, all things considered, defeated by the reasons against its performance. I argue that such a view can only generate appropriate substantive constraints in the context of criminalisation if it can distinguish between the sorts of reasons that (...) a verdict of wrongfulness, as a concept distinct from stupidity or selfishness, should attend to, and the sorts of reasons it should leave out. Assuming that this conception of wrongfulness should operate as a constraint on criminalisation in a liberal-democratic state, the only reasons it should include are other-regarding reasons. What matters is whether the agent commits an other-regarding wrong. This conception of wrongfulness helps us further to resolve fundamental questions concerning mala prohibita and the legitimate reach of any duty to obey the law. (shrink)