The development of models to assess the emission control requirements of primary precursor pollutants in the production of photochemical oxidants has been underway for approximately 20 years. Over the period there has been a considerable increase in our understanding of the basic scientific processes associated with the production of photochemical oxidants in the atmosphere and a concomitant advance in the analytical sophistication by which these processes are incorporated to derive quantitative precursor relationships. The establishment of air quality standards implies the use of such quantitative relationships to describe the link between emissions and air quality in order to consider an effective and cost efficient control program. The paper maps out the progress that has been made in the endeavor, discussing the successes and shortcomings of the present generation of modeling tools. Model performance evaluations, accuracy, uncertainty and sensitivity analyses are also discussed.