Authentication is the backbone of network security. It is the process by which a system verifies the identity of a user before granting access to secure resources. In theory, authentication should be a simple, binary decision: either the user is who they claim to be, or they are not. However, the reality is far more complex, particularly when the methods of authentication are built on flawed principles. This chapter explores the science behind authentication, the binary nature of computer science, and how current authentication methods fall short.