The rate and mechanism of chemical change has been studied for many years but until recently experimental work has been concerned with bulk systemv so that the microscopic level of action was manifest only in some average way. In contrast, methods such as X-ray scattering yield direct information on molecular structure. The technique of molecular beam scattering is now a similar tool, capable of probing the dynamics of chemical reaction at the molecular level and leading to an understanding of the process in terms of the trajectories of the atoms participating. Discussion of the reaction is thus naturally in quantum channel cross-section terms; a ohange in emphasis, from conditions in the bulk sample to the collision proper, which has stimulated theoretical work. In particular, concepts and techniques developed for use in the atomic and nuclear collision field have been extended into this area. These rather rigorous calculations will be limited to the very simplest systems but, nevertheless, will form an important proving ground for more approximate models.