The mechanisms of proteolysis remain to be fully defined. This review focuses on recent advances in our understanding of the ubiquitin-proteasome-dependent pathway, which is involved in the control of many major biological functions. The ubiquitinylation/deubiquitinylation system is a complex machinery responsible for the specific tagging and proof-reading of substrates degraded by the 26S proteasome, as well as having other functions. The formation of a polyubiquitin degradation signal is required for proteasome-dependent proteolysis. Several families of enzymes, which may comprise hundreds of members to achieve high selectivity, control this process. The substrates tagged by ubiquitin are then recognized by the 26S proteasome and degraded into peptides. In addition, the 26S proteasome also recognizes and degrades some non-ubiquitinylated proteins. In fact, there are multiple ubiquitin- or proteasome-dependent pathways. These systems presumably degrade specific classes of substrates and single proteins by alternative mechanisms and could be interconnected. They may also interfere or cooperate with other proteolytic pathways.