The ubiquitin-proteasome system degrades an enormous variety of proteins that contain specific degradation signals, or 'degrons'. Besides the degradation of regulatory proteins, almost every protein suffers from sporadic biosynthetic errors or misfolding. Such aberrant proteins can be recognized and rapidly degraded by cells. Structural and functional data on a handful of degrons allow several generalizations regarding their mechanism of action. We focus on different strategies of degron recognition by the ubiquitin system, and contrast regulatory degrons that are subject to signalling-dependent modification with those that are controlled by protein folding or assembly, as frequently occurs during protein quality control.