Anonymity and unobservability have become key issues in the context of securing privacy on the Internet and in other communication networks. Services that provide anonymous and unobservable access to the Internet are important for electronic commerce applications as well as for services where users want to remain anonymous. This book is devoted to the design and realization of anonymity services for the Internet and other communcation networks. The book offers topical sections on: attacks on systems, anonymous publishing, mix systems, identity management, pseudonyms and remailers. Besides nine technical papers, an introduction clarifying the terminology for this emerging area is presented as well as a survey article introducing the topic to a broader audience interested in security issues.