Ribonuclease P (RNase P) is an essential endonuclease that acts early in the tRNA biogenesis pathway. This enzyme catalyzes cleavage of the leader sequence of precursor tRNAs (pre-tRNAs), generating the mature 5' end of tRNAs. RNase P activities have been identified in Bacteria, Archaea, and Eucarya, as well as organelles. Most forms of RNase P are ribonucleoproteins, i.e., they consist of an essential RNA subunit and protein subunits, although the composition of the enzyme in mitochondria and chloroplasts is still under debate. The recent purification of the eukaryotic nuclear RNase P has demonstrated a significantly larger protein content compared to the bacterial enzyme. Moreover, emerging evidence suggests that the eukaryotic RNase P has evolved into at least two related nuclear enzymes with distinct functions, RNase P and RNase MRP. Here we review current information on RNase P, with emphasis on the composition, structure, and functions of the eukaryotic nuclear holoenzyme, and its relationship with RNase MRP.