Ribozymes are an important new class of metalloenzymes that have an unlikely feature: they are made entirely of ribonucleic acid (RNA). Metal ions are essential for efficient chemical catalysis by ribozymes and are often required for the stabilization of ribozyme structure. Most ribozymes catalyze reactions at phosphorus centers through one of two major mechanistic pathways, and reaction has been observed at carbon centers. Creative experiments have revealed the position of metal ions in the active site of two ribozymes. The exploitation of variable metal geometry and reactivity has expanded ribozyme chemistry and has facilitated the application of in vitro selection for the creation of novel ribozymes.