It is now possible to use rigorous quantum scattering theory to perform accurate calculations on the detailed state-to-state dynamics of chemical reactions in the gas phase. Calculations on simple reactions, such as H + D-2 --> HD + D and F + H-2 --> HF + H, compete with experiment in their accuracy. Recent advances in theory promise to extend such accurate predictions to more complicated reactions, such as OH + H-2 --> H2O + H, and even to reactions of molecules on solid surfaces. New experimental techniques for probing reaction transition states, such as negative-ion photodetachment spectroscopy and pump-probe femtosecond spectroscopy, are stimulating the development of new theories.