A recent detailed investigation of the hazard ranges predicted by a variety of computer codes for several types of accident scenario involving 16 hazardous substances highlighted a variety of problems for the unwary computer code user and revealed deficiencies in the codes themselves. The physical phenomena modelled included outflow from pipes and vessels, pool evaporation and dispersion of dense and passive gases. In all, about 2000 computer runs were made to investigate the sensitivity of output parameters (such as mass flow rate, evaporation rate and concentration) to different inputs. This paper discusses the expectations of computer code users, and attempts to show that code producers would have a better working relationship with their customers if they supplied codes that were more tailored to the customers＇ needs.