Reverse Monte Carlo simulations have been carried out to provide molecular-level detail on the structures of three different lipid and surfactant monolayers of particular relevance in the design and development of drugs and drug delivery systems. The simulations were performed using the SURFISS program, with the orientations and conformations of the molecules in the monolayers optimised iteratively using constraints furnished from neutron reflectance-derived volume fraction profile data. The three systems modelled involve the monolayers formed at the air/water interface by the synthetic surfactants N,N-dimethyldodecylamine-N-oxide and 1,2-di-O-octadecyl-rac-glyceryl-3-(ω-methoxydodecaoxyethylene) and the self-assembled layer of dimyristoylphosphatidylcholine adsorbed onto silicon hydrophobized by treatment with octadecyltrichlorosilane.