Internet-scale Identity Management is a fascinating topic with many facets. The prevalent solution today duplicates identity information at many, separate places and uses as many different passwords to access and protect them. Users cannot handle the resulting plethora of credentials and weak security is the result. Plenty of technical approaches to remedy the situation exist, but little of them have managed to offer a complete and secure solution. Above all of them floats the keyword Single Sign-On as a seemingly trivial way out of the mess. But a great number of pitfalls prevented the success of almost all systems proposed so far. This book discusses prominent systems of the past and shows why they have failed to be adapted for the Internet. Based on a study of these deficiencies, we derive basic requirements to be adapted by future systems. An overview and comparison brings light into the jungle of current approaches. Among others, we discuss Windows CardSpace, OpenID, and the Liberty Alliance Project. Practial examples complete the discussions.