The posttranslational modification of proteins by ubiquitin ("ubiquitylation") has taken center stage in eukaryotic cell biology. Ubiquitylation triggers the degradation of damaged proteins, cell cycle regulators, transcription factors and metabolic enzymes by the 26S proteasome. Moreover, it serves as a versatile mark in many non-proteolytic processes such as DNA damage repair, receptor signaling and endocytosis. Importantly, abnormalities of the ubiquitin system causally contribute to the pathogenesis of multiple human diseases including cancer, neurodegenerative disorders and infectious diseases. In many cases, however, neither the precise function of the affected ubiquitin system component in healthy individuals nor details of the pathogenesis following its impairment are known. These limited mechanistic insights constitute an obstacle to the design of efficient therapeutic strategies and emphasize the requirement for continued efforts in basic research.