This book is an introduction to game theory from a mathematical perspective. It is intended to be a first course for undergraduate students of mathematics, but I also hope that it will contain something of interest to advanced students or researchers in biology and economics who often encounter the basics of game theory informally via relevant applications. In view of the intended audience, the examples used in this book are generally abstract problems so that the reader is not forced to learn a great deal of a subject – either biology or economics – that may be unfamiliar. Where a context is given, these are usually “classical” problems.