The Mexico City Air Quality Research Initiative was a 3-yr international collaborative project to develop or adapt a set of air quality management decision analysis tools for Mexico City and make them available to Mexican policy makers. The project comprised three tasks: modeling and simulation, characterization and measurement, and strategic evaluation. A prognostic, mesoscale meteorological model was adapted to the region of Mexico City and linked to a 3-D airshed model. These were extensively tested against data from the air quality monitoring network and from three intensive field campaigns. The interaction between policy and science was promoted through the development of a formal multiattribute decision analysis model to evaluate alternative control strategies. The project benefited by having researchers from both nations working side by side as peers, by having both nations investing resources and having an interest in the success of the project, and by having an objective, not of advocacy, but of the application of science to problem solving.