Catalytic RNAs are biological catalysts, a group of molecules which also comprises a much bigger number of younger and more effective catalysts – the enzymes. The RNA backbone is highly negatively charged with very flexible bonds, and the functional groups are few and monotonous. However, RNA catalyzes a few biochemical reactions that are of central importance for cells, using mostly metal ion catalysis and acid-base catalysis, as well as the contribution of binding energy, electrostatic and covalent catalysis, and even sometimes using organic cofactors and the supstrate itself as a contributing factor in catalysis. Small ribozymes (the hammerhead, hairpin and HDV) are self-cleaving; they activate the 2'-OH group of the ribose adjacent to the scissile phosphate for nucleophilic attack. In large ribozymes (group I and II introns) the nucleophile is the –OH group of the ribose of a distant or even exogenous nucleotide; they catalyze intron splicing. There are also a few ribonucleoprotein complexes in the cell in which RNA is the catalytical entity. The most important is the ribosome. The fact that RNA is a molecule that can contain genetic information and allow genetic continuity, on one hand, and can catalyze chemical reactions, on the other, is supportive of the hypothesis of an RNA world in which, during the beginnings of life on Earth, RNA combined early metabolism and information transfer.