The presented review concerns the structure functional analysis of proteasomes and the participation of ubiquitin proteasome system in the basic cellular processes. 26S proteasomes is a key enzyme of the ubiquitin-dependent pathway of protein degradation in cells. This protein particle is composed of 20S catalytic core and associated regulatory complexes. In addition to several types of peptidase activities, eukaryotic proteasomes also have endoribonuclease, protein-chaperone and DNA-helicase activities. The ubiquitin proteasome system controls the levels of most regulatory proteins in a cell and, thus, is absolutely necessary element for cell life. Proteasomal population in a cell is structurally and functionally heterogeneous. These particles are subjected to multistep regulation, particularly, by set of posttranslation modifications. In this review, we also consider the current knowledge on the involvement of proteasomes in controlling the cell cycle, transcription, apoptosis, differentiation, DNA repair and immune response.