The COP9 signalosome (CSN) is a conserved protein complex that functions in the ubiquitin–proteasome pathway. After two decades of research, we now know that the CSN is a multi-subunit protease that regulates the activity of cullin–RING ligase (CRL) families of ubiquitin E3 complexes. The CSN is rapidly emerging as a key player in the DNA-damage response, cell-cycle control and gene expression. The independent functions of CSN5 (also known as JAB1) add to the complexity of the CSN machinery. Here, we provide an updated view of the structure, functions and regulation of this protein complex.