This chapter sketches how mens rea serves at least five different functions, which can be grouped into two broad categories. Within the first category, it helps to establish that the defendant’s offence was an instance of culpable wrongdoing. It does this in three different ways. First, and most obviously, it contributes to findings of culpability. Secondly, it can help to identify what kind of action a person is performing: in these cases, the finding of mens rea is integral to the moral wrongness of the action for which a person is being held responsible. Thirdly, mens rea affects the availability of justifications. Whether a person’s pro tanto wrong was, all things considered, wrongful—unjustified—and an instance of wrongdoing—depends on the reasons why they did it. The other broad category concerns the principle of legitimate enactment. Mens rea has important roles to play in articulating, and notifying, the limits of citizens’ freedom. More specifically, a fourth function of mens rea is to secure fair warning to defendants, ensuring they have sufficient advance notice that, by their conduct, they risk violating the criminal law. Finally, mens rea plays a key mediating role in criminalization, being part of the trade-off between the protection of potential victims and the preservation of liberties for potential defendants.