The posttranslational modification with ubiquitin, a process referred to as ubiquitylation,
controls almost every process in cells. Ubiquitin can be attached to substrate proteins as a
single moiety or in the form of polymeric chains in which successive ubiquitin molecules are
connected through specific isopeptide bonds. Reminiscent of a code, the various ubiquitin
modifications adopt distinct conformations and lead to different outcomes in cells. Here, we
discuss the structure, assembly, and function of this ubiquitin code.