We make choices all the time — about trivial matters, about how to spend our money, about how to spend our time, about what to do with our lives. Choice Theory: A Very Short Introduction introduces ideas from economics, philosophy, and other areas, showing how the theory applies to everyday decisions, and to particular situations such as gambling and resource allocation. But what kind of criteria are we applying when we say that a choice is rational? What guides our own choices? What strategies should be applied in making decisions that affect a lot of people? This VSI explores what it means to be rational in all these contexts.