Realism is a theory that claims to explain the reality of international politics. It emphasises the constraints on politics that result from humankind’s egoistic nature and the absence of a central authority above the state. For realists, the highest goal is the survival of the state, which explains why states’ actions are judged according to the ethics of responsibility rather than by moral principles. The dominance of realism has generated a significant strand of literature criticising its main tenets. However, despite the value of the criticisms, which will be explored in the rest of this book, realism continues to provide valuable insights and remains an important analytical tool for every student of International Relations.