Proteasomes are structures responsible for the elimination of damaged and misfolded proteins. Thus, they also regulate the most important intracellular processes. Changes in their functions can lead to many molecular diseases. There are two possible disorders in the function of proteasomes. Their increasing activity causes excessive degradation of important cell proteins. On the other hand, their insufficiency can inhibit the degradation of pathological proteins and lead to their accumulation. The increase of proteasome activity and the degradation of important proteins are observed in many pathological disorders. Therefore the study of pharmacological methods using proteasome inhibitors has gained growing interest in the last years. This review summarizes recent findings regarding the role of proteasomes in pathogenesis of selected diseases and discusses the potential use of proteasomes in diagnosis of different disorders.