The ubiquitin system plays an essential role in numerous cellular processes by controlling protein stability and function. A deregulation of this system has been reported in various pathologies including cancer, neurodegenerative diseases and immune disorders. Most of the enzymes involved in adding or removing ubiquitin chains have been identified, but often their direct substrate and the type of ubiquitylation remains to be clarified. A better understanding of the mechanisms governing these processes is likely to allow the identification of novel targets for pharmacological intervention and pave the way for improved therapies. The latest developments in this rapidly moving field are presented in this book.