Abstract Recent progress in the theoretical description of the interaction between molecules and metal surfaces and the reactivity of metal surfaces and of chemisorbed molecules is described. Electronic factors affecting associative and dissociative chemisorption, sticking, surface chemiluminescence and other surface reactions are illustrated with a few examples. In these dynamical processes several important features can be distinguished: the downward shift of molecular affinity levels upon approaching the surface and the fact that the active region usually lies outside the static equilibrium position studied by common spectroscopies. Further, the high reactivity of metals can in several cases be traced back to the fundamental and unique property of metals having a quasi-continous spectrum of electronic excitations. The qualitative concepts and considerations are based on results of self-consistent calculations on simple models that stress the extended nature of the conduction-electron states. In a few cases the implications have been tested by dynamical experiments.