This chapter deals only with national models, and, in fact, almost all the works referred to are national water use rather than national water demand studies. There exists a large number of exercises aimed at one or another of the model purposes baseline forecasting; the analysis of the impact of government policies; or the balancing of water use and supply. In the sections that follow, each of these purposes is discussed in turn, with reference where appropriate to a sample of studies from a variety of countries.