This book deals with the number system, one of the basic structures in mathematics. It is concerned especially with way of classifying numbers into various categories; for example, it provides some criteria for deciding if a given number is rational (i.e., representable as a common fraction) or irrational, if it is algebraic or transcendental. In the course of the later chapters, the reader is introduced to some of the more recent developments in mathematics. Professor Niven's book may be read with profit by interested high school students as well as by college students and others who want to know more about the basic aspect of pure mathematics. Most readers will find the early chapters well within their grasp while ambitious readers will profit by the more advanced material to be found in later chapters.