Intended to serve as a one-semester introductory course in number theory, this second edition has been revised throughout. In particular, the field of cryptography is highlighted. At the heart of the book are the major number theoretic accomplishments of Euclid, Fermat, Gauss, Legendre, and Euler. In addition, a wealth of new exercises have been included to fully illustrate the properties of numbers and concepts developed in the text. The book will serve as a stimulating introduction for students new to number theory, regardless of their background.