In Quebec's municipal taxation system, the tax burden is allocated according to the market value of properties. Determining the market values for housing inventories containing tens of thousands of properties is obviously a complex task with enormous implications in terms of the resources required and the quality of the resulting estimates. This article presents PariTOP, a decision-support system that uses mathematical programming in general and goal programming in particular to estimate the market value of large numbers of residential properties. PariTOP is the result of a partnership between the Communaute urbaine de Quebec (CUQ), an administrative unit that comprises 12 cities, and Modellium, a Canadian company that develops decision-support systems.