An account of the use of stable isotopes (2H, 13C, 15N, 18O, 34S) for the study of organic and organometallic reaction mechanism and structure, by way of kinetic determinations and modelling, NMR spectroscopy and mass spectrometry is presented. The techniques discussed include use of kinetic isotope effects, label-facilitated nOe experiments, cross-over experiments, stereoisotopochemical analysis, label-facilitated NMR analysis of enantiomers using chiral-shift reagents and NMR in chiral liquid crystal matrices. The reactions in which these tools have been applied are ß-H elimination in Pd-s-alkyl complexes, the Bayliss–Hilman reaction, C–H insertion of stable carbenes, ring-closing metathesis reactions, palladium-catalysed hydrosilyation, palladium-catalysed cycloisomerization, anionic thia-Fries rearrangements, synthesis of proton sponges and their nitrogen stereodynamics, and transition metal (Pd, Mo) catalysed allylic alkylation