The term 'validation' is used ubiquitously in association with the mod
elling activities of numerous disciplines including social, political,
natural, physical sciences, and engineering. There is however, a wide
range of definitions which give rise to very different interpretation
s of what activities the process involves. Analyses of results from th
e present large international effort in modelling radioactive waste di
sposal systems illustrate the urgent need to develop a common approach
to model validation. Some possible explanations are offered to accoun
t for the present state of affairs. We believe that a rigorous approac
h to validation must necessarily be based on a thorough understanding
and application of the theory of simulation and modelling. The methodo
logy developed treats model validation and code verification in a syst
emic and systematic fashion. In fact, this approach may be regarded as
a comprehensive framework to assess the adequacy of any simulation st
udy.