This is a textbook for a course in the theory of games. It is intended for advanced undergraduates and graduate students in mathematics and other quantitative disciplines, e.g., statistics, operations research, etc. It treats the central topics in game theory and is meant to give students a basis from which they can go on to more advanced topics. The subject matter is approached in a mathematically rigorous way, but , within this constraint, an effort is made to keep it interesting and lively. New definitions and topics are motivated as thoroughly as possible. The mathematical prerequisites for understanding the book are modest: basic probability together with a little calculus and linear algebra. Among others, two topics of great current interest are discussed in this book. The idea of iterated Prisoner's Dilemma (super games) is considered. It is specially of great interest to biologists, sociologists and others who use it in studying the evolution of cooperative behavior both in nature and in human society. Also covered are challenging game-playing computer programs.