Trend of the concentrations of air pollutants in the Seoul Metropolitan Area (SMA) in the Republic of Korea, mostly in Seoul, is shown and discussed along with the applied policy actions and remaining challenges are identified and further research directions are discussed. It was found that the policies adopted from the developed countries, notably direct emission control measures such as fuel sulfur content control and tightening of the emission standards, have been successful in reducing primary air pollutants such as carbon monoxide, sulfur dioxide, and lead. However, these policies have not been effective in controlling the increased number of emission sources and secondary air pollutants such as particulate matter with an aerodynamic diameter of less than or equal to a nominal 2.5 µm (PM2.5) and ozone. It is recommended (1) to develop reliable emission inventory, (2) to reduce uncertainties on the regional contribution on the air quality in Seoul, and (3) to understand major chemical pathways of ozone and secondary aerosols to develop effective control policies against air pollution. Research directions to accomplish these goals are suggested.