The solid-state provides some unique possibilities of determining reaction parameters. Thus, studies of reactions associated with single crystal-single crystal transformations, and highly refined X-ray diffraction analyses of electron distribution in the parent molecules, provide information on the reaction's driving force and the atomic movement involved. The stereochemical course of the reaction is controlled by the interactions of the reactant molecules with their environment, and therefore by the local structure at the reaction site. This control by structure shows that at least up to the transition state the reactant molecule feels the ground state enviroment; thus we can consider the product-forming step as taking place in solid solution. Depending on the rigidities of the product molecule and of the parent crystal structure there may or may not occur, subsequently, crystallization of the product in its own structure. This corresponds to heterogeneous or homogeneous reaction, respectively. The texture of the resultant solid may be of technical significance.