Ribonuclease P (RNase P) is an endoribonuclease that catalyzes the processing of the 5’ leader sequence of precursor tRNA (pre-tRNA). Two distinct forms of RNase P have been identified in nature. The first is RNA-based RNase P, composed of a single catalytic RNA component and a variable number of proteins. The second is protein-based RNase P, which evolved independently on two separate occasions: a eukaryotic enzyme termed PRORP and a bacterial/archaeal variant termed HARP. The remarkable diversity in enzyme architecture among RNase P enzymes—despite their catalysis of identical reactions—makes them excellent models for understanding enzymatic diversity and molecular evolution. Here, we summarize the history of RNase P enzyme discovery and review the current state of knowledge regarding their structure and function. Additionally, we describe the phylogenetic distribution and molecular evolution of these fascinating enzymes.