The paper gives results from an ongoing National Acid Precipitation Assessment Program (NAPAP) to significantly improve engineering cost estimates currently being used to evaluate the economic effects of applying SO2 and NOx controls to existing coal-fired utility boilers. Initially, procedures were developed to: select plants with boilers representative of the population, conduct plant visits and collect site-specific data, develop boiler/control-specific retrofit difficulty factors, and develop boiler/plant-specific cost and performance estimates for the SO2 and NOx controls selected for evaluation. Eleven commercial and developmental SO2 and NOx control technologies were selected for inclusion in the program. Of 30 'top 200' plants selected for detailed study, 12 have been visited. Based on results from these 12 plants, simplified procedures have been developed and used to evaluate about 50 of the remaining top 200 plants.