A model used to find the best possible choice out of a set of alternatives. A simple example might be finding the most efficient transport pattern to carry commodities from the point of supply to the markets, given the volumes of production and demand, and transport costs. See Ward et al. (2003) Annals Reg. Sci. 37 on evaluating urban change. Kuby et al. (2005) Adv. Water Resources 28, 8 introduce a new class of combinatorial-optimization models to appraise dam removal.