When two or more small molecules combine in a crystal, unusual opportunities may arise. The rewards that result from such opportunities include chemo-, stereo- and enantiospecific syntheses, the synthesis of polymer single crystals, and the facile preparation of otherwise-unattainable molecules. Beyond the rewards lies a challenge: the discovery and design of new classes of materials capable of solid-state reactivity, and the elucidation of the principles which govern their behavior. Pursuit of this goal requires knowledge of molecular solid-state chemistry, principles of crystal packing, and chemical reactivity. Salts and complexes of metals with pendant unsaturated carboxylates provide a set of reactive centers which may be proximate in the solid state. In favorable cases, a set of "favorable" intermolecular contacts will satisfy the criteria for the onset of solid state reactivity, viz., that unsaturated groups be oriented in a parallel fashion, at distances ca. 4.2 Å from one another. Recent studies also include: (a) discovery of new polymorphs of important pharmaceuticals, (b) new methodologies for producing “movies” of the molecular motion which occurs in a phase transition or solid-state reaction, and (c) novel approaches to the understanding of twinning phenomena in crystals. All the topics briefly outlined here are illustrated in the selected publications.