The discussion will center around ribonuclease P (RNase P), an ubiquitous, essential enzyme that has an RNA subunit. In eubacteria, the RNA subunit is catalytic and there is one protein subunit that acts as a cofactor. In eukaryotes, the RNA subunit is not catalytic and there are several protein subunits. The properties of the enzyme and the subunits from these two kinds of organisms will be discussed as well as the relevance of the finding of catalytic RNA for theories of the origin of life. A new, general method for the inactivation of gene expression, based on the properties of RNase P, will be outlined. This method has been applied successfully in laboratory cultures to a variety of clinically important problems.