The two most important questions that network security protocols attempt to answer are "Who are you?" and "Should you be doing that?" This lecture will give an overview of how network security protocols work, including the basics of Cryptography, key distribution, and protocol design pitfalls. It also will emphasize the system issues involved in making such systems work. Although the cryptography might be sound and the protocols themselves correct, there tend to be other issues, without which things are not deployable, or scalable, or secure. The challenges will be emphasized, as well as the deployed and standardized technology. Once the generic issues are covered, there will be an overview of deployed and emerging standards such as Kerberos, X.509, IPsec, and SSL.