Proteasomes are large multicatalytic proteinase complexes located in the cytosol and the nucleus of eukaryotic cells. The ubiquitin-proteasome system is responsible for the degradation of most intracellular proteins and therefore plays an essential regulatory role in critical cellular processes including cell cycle progression, proliferation, differentiation, angiogenesis and apoptosis. Besides involving in normal cellular functions and homeostasis, the alteration of proteasomal activity contributes to the pathological states of several clinical disorders including inflammation, neurodegeneration and cancer. It has been reported that human cancer cells possess elevated level of proteasome activity and are more sensitive to proteasome inhibitors than normal cells, indicating that the inhibition of the ubiquitin-proteasome system could be used as a novel approach for cancer therapy. In this review we summarize several specific aspects of research for the proteasome complex, including the structure and catalytic activities of the proteasome, properties and mechanisms of action of various proteasome inhibitors, and finally the clinical development of proteasome inhibitors as novel anticancer agents.