The two types of proteasomes with apparent sedimentation coefficients of 20S and 26S consist of a number of heterogeneous polypeptides and are unusually large protein complexes of approximately 750 kDa and 2000 kDa, respectively. The 26S proteasome is a cylindrical caterpillar-shaped complex with a symmetrical assembly of a four-layered central 20S proteasome and two terminal 22S regulators each with a V-like structure. The central core and the terminal structures are formed by multiple polypeptides with molecular masses of 21-31 kDa and 28-112 kDa, respectively. We have been studying their detailed structures by protein-chemical and molecular biological techniques. In this review, we summarize the structural features of eukaryotic 20S and 26S proteasomes. We also discuss the possible function(s) of the terminal multi-protein regulator complex based on current information.