This groundbreaking text examines the problem of user authentication from a completely new viewpoint. Rather than describing the requirements, technologies and implementation issues of designing point-of-entry authentication, the book introduces and investigates the technological requirements of implementing transparent user authentication - where authentication credentials are captured during a user's normal interaction with a system. This approach would transform user authentication from a binary point-of-entry decision to a continuous identity confidence measure. Topics and features: discusses the need for user authentication; reviews existing authentication approaches; introduces novel behavioural biometrics techniques; examines the wider system-specific issues with designing large-scale multimodal authentication systems; concludes with a look to the future of user authentication.