This chapter examines why states pursue power from the perspective of structural realism. It considers a body of realist theories that argue that states have deep concern for the balance of power and compete among themselves either to gain power at the expense of others, or at least to make sure they do not lose power. This competition for power makes for a dangerous world where states sometimes fight each other. There are, however, important differences among structural realists. The chapter first explains why states want power and how much power they want before discussing the causes of war. These theoretical issues are illuminated with a case study that assesses whether China can rise peacefully according to offensive realism vs. defensive realism. Along the way, concepts such as the security dilemma, offence–defence balance, central war, buck-passing, unipolarity, bipolarity, and multipolarity, and deterrence are analysed.