This paper summarizes a number of investigations of reactions occurring in organized monolayer assemblies, monolayer films, and at monolayer assembly or film-solution interfaces. In several cases, the reactions involve chromophores localized in relatively hydrophobic sites and pronounced differences in reactivity can be ascribed to the microenvironment. Metallation of free base porphyrins has been examined in assemblies and at film-solution interfaces. Although several divalent metal ions can be incorporated, the order of reactivity varies from that observed in solution. Several porphyrins residing in highly hydrophobic sites are resistant to metallation while those lying near the hydrophilic interface react rapidly. Photooxidation of surfactant protoporphyrin esters residing in hydrophobic regions occurs in both micelles and monolayers but the product distribution is quite different from that obtained in solution. Iron and ruthenium porphyrins isolated in assemblies undergo ligand substitution reactions not readily observable in solution. The photoreactions of some surfactant styrenes has been examined; close packing of the styrene unit, which lies at the end of a hydrocarbon chain, leads to substantial ground state interaction and rapid and selective dimerization on irradiation. Different products are obtained from those formed on irradiation of the same compound in solution, micelles, and crystals. The photoreactivity of a surfactant phenyl ketone has been investigated in condensed monolayers. In solution the primary reaction of the ketone is the “type II” photoelimination which proceeds via α-hydrogen abstraction. In condensed monolayers the required conformation for this reaction can evidently not be readily attained. Thus in the assemblies dramatic reduction in the efficiency of the type II process occurs concurrent with the onset of new reaction paths.