Research on the activity of self-cleaving proteases in bacterial, mammalian and virus-infected cells is reviewed, with an emphasis on the diversity of regulatory systems controlled by protein processing. Each of these three groups will be considered in turn by focusing on the following systems: the Rec A-dependent intramolecular cleavage of the Escherichia coli SOS response protein, LexA; the intramolecular activation of the mammalian aspartic acid protease, pepsinogen; and the autocatalytic cleavage of polyproteins synthesized by picornaviruses.