Abstract. Chemical reactions occur at the smallest of dimensions, where bonds cleave and form. The size of molecules, catalytic structures, and their containers matters at this nanometer scale.  Diversity and specificity in catalysis exploit size to extend the properties of elements from those in their bulk state.  Reactivity in metals and oxides changes markedly as coordination and electronic environment at exposed surfaces vary with cluster size.  Low-coordination atoms on small clusters stabilize transition states for reactions limited by bond cleavage on bare surfaces. Such atoms, however, also stabilize chemisorbed reactants, making small clusters less reactive when steps require such species.  On semiconductors, such as oxides and sulfides, the charge delocalization required at transition states leads to an intrinsic link between reactivity and electronic/optical properties.  Confinement of catalytic structures within small voids preserves their size, protects them from impurities, and allows preferential access by certain reactants, while also selecting specific transition states, thus conferring enzyme-like specificity to chemical catalysis.