This chapter offers responses to the question ‘why regulate?’ and ‘why do regulatory regimes emerge in a particular form?’ by examining ‘theories of regulation’. While chapter 1 introduced the readers to the economic justifications of regulation, this chapter delves into the different theories that explain why we need regulation and how public and private actors interact to shape the content of regulation. These theories refer to a set of propositions or hypotheses about why regulation emerges, which actors contribute to that emergence, and typical patterns of interaction between regulatory actors. It discusses theories from several disciplinary approaches, classifying these theories into four kinds: public interest, private interest theories, systems and institutionalist approaches and ‘hybrid’ theories.