The article reviews the use of isotopes to determine mechanisms of homogeneously catalyzed reactions. Kinetic isotope effects (KIEs) and equilibrium isotope effects (EIEs) on single steps as well as those on complete catalytic cycles are considered. Coverage includes EIEs and KIEs for the addition and elimination of H
2
and hydrocarbons to metal complexes. Isotope effects for proton, hydrogen atom, and hydride transfer to and from metal complexes, along with insertions into metal–hydrogen bonds, are discussed in detail. Isotope effects on olefin polymerization, oxidation, hydrogenation, and hydroformylation are discussed. Isotope effects on spectroscopic properties are included by way of discussion and interpretation.