This paper suggests that model validation and model legitimisation are
two overlapping but nevertheless distinct activities, and that it tak
es more than being valid for an OR model to be organizationally accept
able: it has to be legitimate. The paper forwards the idea that the im
plementation of a model is necessarily part of a change process and he
nce has impact on the organization. It discusses how organizational co
ntract and legitimacy are related to one another, and how, in a change
process, a model can be used in different modes by different stakehol
ders. Concrete suggestions for model legitimisation are proposed. The
paper concludes by linking model legitimisation and model validation.