Ribonuclease P (RNase P) from Escherichia coli or its catalytic RNA subunit can efficiently cleave small RNA substrates that lack the conserved features of natural substrates of RNase P if an additional small RNA is also present. This additional RNA must contain a sequence complementary to the substrate [external guide sequence (EGS)] and a 3′-proximal CCA sequence to ensure cleavage. The aminoacyl acceptor stem and some additional 5′- and 3′-terminal sequences of a precursor transfer RNA are sufficient to allow efficient cleavage by RNase P, and the 2′-hydroxyl group at the cleavage site is not absolutely necessary for cleavage. In principle, any RNA could be targeted by a custom-designed EGS RNA for specific cleavage by RNase P in vitro or in vivo.