The 26S proteasome is a non-lysosomal protease in the cytosol and nucleus of eukaryotic cells. Its main function is to mediate ubiquitin-dependent proteolysis. The 26S proteasome is a multimeric complex composed by the 20S proteasome CP (core particle) and the 19S RPs (regulatory particles). Although the atomic structure of the 26S proteasome has not yet been determined, high-resolution structures are available for its CP. Studies on the complicated assembly pathway of the proteasome have revealed that it involves an unprecedented number of dedicated chaperones. Assembly of the CP alone involves three conserved proteasome-assembly chaperones [PAC1–PAC2, PAC3–PAC4 and UMP1 (ubiquitin-mediated proteolysis 1)]. Whereas the two heterodimeric PACs have been implicated in the formation of rings of the seven distinct α subunits, UMP1 is important for the formation and dimerization of proteasome precursor complexes containing β subunits. Dimerization coincides with the incorporation of the last β subunit (β7). Additional modules important for the assembly of precursor complexes and their dimerization reside in the β subunits themselves, either as transient or as permanent extensions. Particularly important domains are the propeptide of β5 and the C-terminal extensions of β2 and β7. Upon maturation of the active sites by autocatalytic processing, UMP1 is degraded by the native proteasome.