This text teaches students about the role of number theory in pure mathematics and its important applications to cryptography and other areas. Designed for a one-semester undergraduate course in number theory, the book offers many pedagogical features. Check your understanding problems assess whether students have learned essential information. At the end of every chapter, exercises reinforce an understanding of the material. Other exercises introduce new and interesting ideas while computer exercises reflect the kinds of explorations that number theorists often carry out in their research.