This chapter outlines classifications of user-authentication methods based on five different taxonomies. The outlined taxonomies are: user identification-based taxonomy, authentication methodology-based taxonomy, authentication quality-based taxonomy, authentication complexity-based taxonomy, and authentication scope-based taxonomy. Administration of user authentication is briefly discussed. A technical survey of different user-authentication methods and elements of a user-authentication method is also given. User-identification methods such as passwords, tokens, and biometric methods are described. Proposed and implemented user-authentication protocols to overcome security weaknesses of the basic password authentication are surveyed. New user-authentication protocols such as PANA and SOLA are also included. Protocols and software for user registration as well as centralized user authentication are presented and compared.