With the rise of the Internet, computer systems appear to be more vulnerable than ever from security attacks. Much attention has been focused on the role of the network in security attacks, but it is ultimately the computer operating system that is compromised as a result of these attacks. The computer operating system represents the last line of defense in our security chain. This final layer of defense and its core defense mechanisms of authentication, authorization, and auditing deserve closer scrutiny and review. This chapter will provide an exploratory, descriptive, and evaluative discussion of these security features in the widely used Windows and Linux operating systems.