Companies and institutions depend more than ever on the availability of their Information Technology, and most mission critical business processes are IT-based. Business Continuity is the ability to do business under any circumstances and is an essential requirement faced by modern companies. Both concepts - High Availability and Disaster Recovery - are realized by redundant systems. This book presents requirements, concepts, and realizations of redundant systems on all abstraction levels, and all given examples refer to UNIX and Linux Systems.