AAA ATPases are found in all living organisms. Their common feature is the presence of a highly conserved the AAA domain referred to as AAA module that is responsible for ATP binding and hydrolysis. The AAA domain is required for proper function of AAA proteins. It contains 200-250 residues, among them there are two classical motifs, Walker A (GX4GKT) and Walker B (HyDE). AAA proteins participate in variety of cellular processes, including cell-cycle regulation, protein proteolysis and disaggregation, organelle biogenesis and intracellular transport. Some of them function as molecular chaperones, subunits of proteolytic complexes or independent proteases (FtsH, Lon). They also act as DNA helicases and transcription factors. This review describes the structure, function and mechanisms of action of the most known AAA proteins.