Using data from 1995 and 1996, the authors of this paper estimate the cost of the Arizona Enhanced I/M Program and the emission reductions achieved. They begin by enumerating briefly the components of I/M costs and discuss their size and incidence. Then they describe the empirical information from Arizona and how they use it to construct cost estimates for both vehicle inspection and repair of failing vehicles. Inspection costs include the costs of operating the test stations and the costs motorists incur in time and money to get to the station and go through the testing process. The authors find that the inspection costs account for over two-thirds of the full costs of I/M, while costs associated with actual vehicle repair account for only one third. They conclude by comparing the empirical estimates of costs and program effectiveness in the Arizona program with the ex ante estimated Enhanced I/M program costs made by the EPA in the 1992 Regulatory Impact Analysis (RIA). The ex ante EPA analysis appears to have underestimated the costs of achieving the ambitious reductions in emissions hoped for under I/M.