A large proteolytic complex that degrades intracellular proteins. Intracellular proteins are continuously synthesized and degraded, and their levels in cells reflect the fine balance between these two processes. The rate of breakdown of individual proteins inside the cell varies widely and can be altered according to changes in the cellular environment. In eukaryotic cells, the site for degradation of most intracellular proteins is a large proteolytic particle termed the proteasome. Proteasomes are a major cell constituent, constituting up to 2% of cellular protein, and are essential for viability. They are found in the cytoplasm and nucleus of all eukaryotic cells. Simpler but homologous forms of the proteasome are also present in archaea and bacteria. Much of the knowledge about the structure and function of eukaryotic proteasomes has been gained from studies of these simpler systems.