This book focuses on a single problem: how to factor a large integer or prove its prime. From the Sieve of Eratosthenes of ancient Greece to the Multiple Polynomial Quadratic Sieve and the Elliptic Curve Methods discovered in the past few years, this self-contained text provides a survey of the heritage and an introduction to the current research in this field. It can also be used as an introduction to Number Theory and has the advantage over most texts in this area of being built around a unifying theme. With its strong emphasis on algorithms, it encourages learning through computation and experimentation.