General characteristics and trends of ground-level ozone were investigated by using observations from over 80 stations in Korea for the period of 1990--1995. In general, seasonal ozone variations are characterized by a spring peak with decreased levels in the summer, followed by increases in the fall. In summer, owing to frequent precipitation, monthly average ozone levels notably decreased in most coastal areas, while a weak depression was observed in inland stations, sustained by photochemical ozone buildup under relatively low wind speeds. Coupled with frequent precipitation in summer, with high ozone-forming potential, low wind speeds in inland areas play a significant role in local variations of ozone concentrations. Accordingly, ozone concentrations vary in a quite different manner among monitoring stations in the Greater Seoul Area, as well as among major cities nationwide. On the whole, monthly average ozone concentrations in major cities, whether located in inland or coastal areas, are about 10--20 ppb lower than background levels of 30--40 ppb typical of the Northern Hemisphere. These lower concentrations are likely due to the titration effect of local NOx emissions, which is a clear indication of urban characteristics for most of the stations. Next, ozone concentration variations at 34--37 stations in the Greater Seoul Area, characterized by typical photochemical air pollution, were closely investigated. Special efforts were made to clarify the effects of transport within the Greater Seoul Area on the ozone concentration variations, a long-standing question at issue, by examining monthly variations of percentile values and frequency distributions of hourly ozone concentrations. It is believed that complex local variations across the nation are superimposed on underlying effects from the Asian continent and the ocean, located to the west and northeast of the Korean Peninsula, respectively. Further research is needed to quantify the relative importance of these effects in controlling the nationwide distribution of ozone levels. ¿ 2000 American Geophysical Union