As the survey of model building in Ireland (Fanning and Bradley, 1982) identified most of the Irish macromodels are short-term in nature. One model, COMET — 1976 (Barten et al., 1976), was an important attempt to build a medium-term model, but a number of models had features which make them to varying degrees, suitable for medium-term analysis. The concern of this paper is with the design of such a policy analysis model. In short-term modelling a fairly ad hoc methodological approach is often acceptable since inconsistencies or systematic divergencies from underlying trends are less important in the short-term. However, medium-term modelling brings into play the entire model structure in a more fundamental way and systematic changes in behaviour lie at the very heart of the analysis. To illustrate this we examine, in the next subsection, one of the larger models of the Irish economy from a medium-term perspective.