Ribonuclease P (RNase P) is a ribonucleoprotein comprised of a catalytic RNA subunit and one or several protein subunits. RNase P is best known for its role in 5'-processing of tRNA precursors. RNase P enzymes from almost all forms of life, including protein-synthesizing organelles, contain an RNase P with a conserved, homologous RNA. Five distinct structure classes of RNase P RNAs have been identified in bacteria and archaea; eukaryotic RNase P RNAs are not yet sufficiently well surveyed for structure classes to be defined. Here we will examine the structure variations in RNase P RNAs in bacteria, archaea, eukaryotes, plastids and mitochondria with special emphasis on the functional roles these unique secondary structures perform.