The measurement of diffuse scattering data over large volumes of reciprocal space is now relatively routine and this opens up the possibility of analysing the local atomic and molecular structure of crystals in a level of detail not possible via conventional crystallography. We describe the development of a general method by which such diffuse scattering data can be analysed. This involves Monte Carlo simulation of a model crystal from which diffraction patterns may be calculated and compared to the observed data. The method is illustrated using two contrasting organic molecular crystal examples. For benzil, C14H10O2, the diffuse scattering is purely thermal in origin while ClMe (p-methyl-N-(p-chlorobenzylidene) aniline, C14H12ClN) is very disordered with each molecular site being occupied by the molecule in one of four different basic orientations and considerable local relaxation of these basic positions and orientations is present.