Abstract
Over recent years the 20S proteasome has been the subject of intensive research and as a result we have obtained a detailed view of its molecular structure and cellular function. Here I will review our knowledge of the prokaryotic and eukaryotic 20S proteasome and its activators in prokaryotes. The eukaryotic 19S regulator which assembles with the 20S core to form the 26S proteasome is reviewed in the following contribution (see the chapter by Glickman and Maytal, this volume).