AbstractOne of the hallmarks of life is the widespread use of certain essential ribozymes. The ubiquitous ribonuclease P (RNase P) and eukaryotic RNase MRP are essential complexes where a structured, noncoding RNA acts in catalysis. Recent discoveries have elucidated the three‐dimensional structure of the ancestral ribonucleoprotein complex, suggested the possibility of a protein‐only composition in organelles, and even noted the absence of RNase P in a non‐free‐living organism. With respect to these last two findings, import mechanisms for RNases P/MRP into mitochondria have been demonstrated, and RNase P is present in organisms with some of the smallest known genomes. Together, these results have led to an ongoing debate regarding the precise definition of how “essential” these ribozymes truly are. © IUBMB IUBMB Life, 2012.