This review focuses on recent insights into the mechanisms and the biological functions of the proteasome. This large ATP-dependent proteolytic complex is the main site for protein degradation in mammalian cells and catalyses the rapid degradation of ubiquitinated proteins, and is the source of most antigenic peptides used by the immune system to screen for viruses and cancer. ATP is required to unfold globular proteins to open the gated channel into the 20S proteasome and to facilitate protein translation into it. Inhibitors of its proteolytic activity are widely used as research tools and have proven effective in cancer therapy.