In eukaryotic cells, the protein degradation is a highly regulated and selective process. Membrane-associated or extracellular proteins are degraded in lysosomes, whereas intracellular protein dismantling is primarily non-lysosomal, being realized by complex, not-membrane enclosed and energy-dependent effectors, the proteasomes, localized in both cytoplasm and nucleus. In mammals, the proteasomes constitute a structurally and functionally heterogeneous system, with shared general architecture, different molecular compositions and functions, and tissue-specific distribution. The proteasomes regulate a wide range of cellular processes, including protein quality control, gene transcription, cell cycle and death, while a proteasomal subpopulation, the immunoproteasomes, is responsible for the generation of peptides acting as immunogenic epitopes in antigen presentation to the immune cells. Due to the multitude of the targeted substrates and the involved processes, it is not surprising that alterations in the proteasome functions may play a pivotal role in the pathogenesis of several human diseases, such as solid or hematologic malignancies, neurodegenerative, immune and inflammatory disorders. Enormous benefits are emerging from the identification and clinical use of proteasome inhibitors, exhibiting a broad array of biologic properties and providing new and even unpredictable therapeutic opportunities.