This fascinating article explores the history and development of cryptography. Published in February 2003 by a division of the National Academy of Sciences, the article has substantial information on the applications of encryption in today's wired world. Additionally, it looks back at some of the most important breakthroughs related to number theory, spanning over two millennia. Some mildly technical descriptions of cryptographic techniques are included, but they are suitable for most audiences. Although the article can be downloaded as a single file, the online version includes an integrated glossary that defines many terms appearing in the text; users can simply click on the hyperlinked word to see the definition. [CL]