The primary translation product of the picornavirus genome is a single large protein which is processed to the mature viral polypeptides by progressive, co- and post-translational cleavages. Replication of the picornaviruses is thus entirely dependent upon the proteolysis of viral precursor proteins. In poliovirus, two virus-encoded proteinases have been identified that catalyze all but the final cleavage of the viral polyprotein. The final processing event, maturation of the virion polypeptide VPO, appears to occur by an unusual autocatalytic serine proteinase-like mechanism. Proteolytic processing of viral precursor proteins is basically similar in all picornaviruses, but recently it has become clear that there are also important differences between these viruses. Understanding of the processing events in picornavirus replication may ultimately lead to the discovery of specific inhibitors of the viral enzymes that could prove clinically useful as anti-viral agents.