Various theoretical perspectives have been developed to explore what type of conduct should or should not be a matter of the law (e.g. see Duff and Green, 2005; Simester and Sullivan, 2007; Hornle, 2014). The main influential theories in the criminal law of Western countries are the Harm Principle, the Offense Principle, and Legal Moralism (including its subset, Moral Paternalism). These theories have been used to examine why certain types of conduct are regulated in different areas of law, including criminal, tort and contract law. This chapter will outline the scope of the Harm Principle, Offense Principle, and Legal Moralism. The following chapters will go on to consider whether the criminalisation of fantasy material can be justified on the basis of any, or all, of these prominent theories.