In photochemical processes, the reactant resides on an excited state potential energy surface and the products accumulate on the ground state. Thus the reaction path must have at least two branches: one located on the excited state and the other located on the ground state energy surface. The novel feature of photochemistry is the "funnel" region where the excited state reactant or intermediate is delivered to the ground state. A modelling strategy for the complete description of what happens at the molecular level from energy absorption to product formation, will be described. The nature of the ground/excited state surface topology that is a requirement for photochromism will be outlinedas well as the current theoretical tools that can be used in modelling. Applications will be briefly reviewed.