Table of contents: 1. Prerequisites and Notation 2. Basic Properties of the Integers 3. Groups, Rings and Ideals 4. Applications to Public Key Cryptography 5. Fields 6. Properties of Finite Fields 7. Applications to Stream Ciphers 8. Boolean Functions 9. Applications to Block Ciphers 10. Number Theory in Public Key Cryptography 11. Where Do We Go from Here?