This classroom-tested, student-friendly text covers a diverse array of number theory topics, from the ancient Euclidean algorithm for finding the greatest common divisor of two integers to recent developments such as cryptography, the theory of elliptic curves, and the negative solution of Hilberts tenth problem. Ideal for a one- or two-semester undergraduate-level course, the second edition features a more flexible structure, adds several new sections, expands exercise sets to encompass a wider variety of problems, and provides calculations, tutorials, and code via an author-maintained website.