Ubiquitin-mediated proteolysis is involved in the turnover of many short-lived regulatory proteins. This pathway leads to the covalent attachment of one or more multiubiquitin chains to target substrates which are then degraded by the 26S multicatalytic proteasome complex. Multiple classes of regulatory enzymes have been identified that mediate either ubiquitin conjugation or ubiquitin deconjugation from target substrates. Timed destruction of cellular regulators by the ubiquitin-proteasome pathway plays a critical role in ensuring normal cellular processes. This review provides multiple examples of key growth regulatory proteins whose levels are regulated by ubiquitin-mediated proteolysis. Pharmacological intervention which alters the half-lives of these cellular proteins may have wide therapeutic potential. Specifically, prevention of p53 ubiquitination (and subsequent degradation) in human papilloma virus positive tumors, and perhaps all tumors retaining wild-type p53 but lacking the retinoblastoma gene function, should lead to programmed cell death. Specific inhibitors of p27 and cyclin B ubiquitination are predicted to be potent antiproliferative agents. Inhibitors of IkappaB ubiquitination should prevent NFkappaB activation and may have utility in a variety of autoimmune and inflammatory conditions. Finally, we present a case for deubiquitination enzymes as novel, potential drug targets.