The proteasome is an essential component of the ATP-dependent proteolytic pathway in eukaryotic cells and is responsible for the degradation of most cellular proteins. The 20 S proteasome whose active site is a threonine,contains multiple peptidase activities. PA700 cappes the 20 S proteasome to form the 26 S complex, by which ubiquitinated proteins are degraded. Advances have been achieved recently in the research about the molecular organization of the 20 S and 26 S particles,their subunits,their intracellular functions and biochemical mechanisms.