This chapter examines the central issues for the justification of criminalization and punishment in the context of criminal law. Specifically, it considers whether there is a class of acts (or omissions) that warrants the use of the label of crime as appropriate. It initially discusses what kind of theory is suitable for grasping and grounding criminalization and punishment, focusing on three types of theory: ideal, situated, and political constructivist. Attention then turns to the central questions to be answered by theories of crime and punishment: what a crime is, what it means to be responsible for a crime, why it is necessary to respond to crime, who may respond to crime, how to respond to crime. Ideas such as retributivism, lex talionis, utilitarianism, and consequentialism are highlighted. The chapter also looks at some influential ways in which the justification of criminalization and punishment has been addressed in post-Enlightenment, occidental thought.