The biological importance of the ubiquitin-proteasome system in the control of myriad cellular processes has been well-recognized; however, the pathophysiological significance of the immunoproteasome, the inducible form of the proteasome, has not been well-comprehended until lately. The primary function of the immunoproteasome was originally believed to improve MHC-I antigen presentation. It now becomes evident that the immunoproteasome possesses broader biological functions. It regulates proinflammatory cytokine production, and T cell differentiation and proliferation. Alongside immune functions, the immunoproteasome has been demonstrated to relieve oxidative stress by the efficient turnover of oxidatively-damaged proteins and by allaying the formation of harmful protein aggregates. Furthermore, it has been implicated to regulate tumor cell growth and control muscle mass. Finally, the immunoproteasome has recently drawn considerable attention as a potential novel therapeutic target for cancer and autoimmune disease. This review will give an overview of the structure and function of the immunoproteasome, highlight its functional diversity in both immune and non-immune responses, and discuss the relationship between the dysregulation of the immunoproteasome and the development of several human diseases.