It is now possible to use rigorous quantum scattering theory to perfor
m accurate calculations on the detailed state-to-state dynamics of che
mical reactions in the gas phase. Calculations on simple reactions, su
ch as H + D-2 --> HD + D and F + H-2 --> HF + H, compete with experime
nt 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 femtosecon
d spectroscopy, are stimulating the development of new theories.