AbstractRecent work on validation has demonstrated the paucity of validation in operational research projects. What little validation is done usually takes place when the modeller builds a model directly for a decision-maker. This paper begins by reviewing and exploring the processes of model-building and usage with different levels of separation between the model-builder and the ultimate user of the model outputs. The different types of validation that these levels of separation imply are then discussed, with emphasis on validation within linear programming. A checklist of validation questions is suggested. New modelling tools coming onto the market affect the levels at which mathematical modelling takes place and alter the ways in which this modelling should best be carried out in business.