Cullin-RING E3 ubiquitin-Ligases (CRLs) are the most prominent class of ubiquitin-ligases. By controlling the stability of a cohort of key regulators, CRLs impinge on many cellular and biological processes such as immunity, development, transcription, cell signalling and cell cycle progression. CRLs are multi-subunit complexes composed of a catalytic site and a substrate recognition module nucleated around a cullin scaffold protein. Most eukaryotic genomes encode at least five distinct cullins, and each of these cullins recruits a specific substrate-recognition module such that CRL complexes are modular. Despite their considerable diversity, CRLs are regulated by similar mechanisms. In particular, recent observations indicate that conformational variability induced by CRL dimerization and by conjugation of the ubiquitin-like protein NEDD8 on the cullin subunit stimulates substrate polyubiquitination. In this review, we discuss the composition of CRL complexes and the various molecular mechanisms controlling their activity.