Choice Theory explores the reasoning which underlies coherent patterns of choice: it explores what it means to act rationally. This book examines what it means to choose rationally in various settings: who to spend the rest of your life with (choice under certainty); whether to undergo an operation if you are told that the mortality rate is 25 per cent, and whether to travel by air in the face of terrorist threats (choice under uncertainty with and without given probabilities); which form to hold your wealth in (the special case of uncertainty where money is involved); whether a country should develop a nuclear capability when it knows that others face the same choice (strategic choice); and the relative advantages of `first past the post` and proportional representation in elections.