Eukaryotic proteasomes are unusually large proteins with a heterogeneous subunit composition and have been classified into two isoforms with apparently distinct sedimentation coefficients of 20S and 26S. The 20S proteasome is composed of a set of small subunits with molecular masses of 21-32 kDa. The 26S proteasome is a multi-molecular assembly, consisting of a central 20S proteasome and two terminal subsets of multiple subunits of 28-112 kDa attached to the central part in opposite orientations. The primary structures of all the subunits of mammalian and yeast 20S proteasomes have been deduced from the nucleotide sequences of cDNAs or genes isolated by recombinant DNA techniques. These genes constitute a unique multi-gene family encoding homologous polypeptides that have been conserved during evolution. In contrast, little is yet known about the terminal structures of the 26S proteasome, but the cDNA clonings of those of humans are currently in progress. In this review, I summarize available information of the structural features on eukaryotic 20S and 26S proteasomes which has been clarified by molecular-biological methods.