We present an analysis of data for tropospheric ozone with a focus on spatial and temporal variations. Surface ozone at mid-latitudes displays two modes of seasonal behavior; a broad summer maximum within a few hundred kilometers of populated and industrialized regions in Europe and the United States and a minimum in summer or autumn in sparsely populated regions remote from industrial activity-in Tasmania and Canada for example. The current data base for different regions, in combination with limited historical data, indicates that summertime concentrations of ozone near the surface in rural areas of Europe and the central and eastern United States may have increased by approximately 6--22 ppb (20%--100%) since the 1940's. The seasonal cycle of ozone in the middle troposphere over Europe, the United States, and northern Japan is very similar to that at the surface with a summer maximum, but it is quite different from that at 300 mbar, which is characterized by a maximum in spring. There is good evidence for an increase in ozone in the middle troposphere over Europe during the past 15 years and weaker evidence for a similar increase over North America and Japan. The increase in tropospheric ozone contributes significantly to the trend in the column of ozone and may compensate for 20%--30% of the decrease in ozone in the stratosphere over middle and high latitudes of the northern hemisphere. We argue that the summer maximum in ozone and the observed trends are due to photochemical production associated with anthropogenic emissions of NOx, hydrocarbons, and CO from combustion of fossil fuels. A strong seasonal variation in ozone observed at Natal, Brazil (6¿S), may also result from emissions of NOx and hydrocarbons, in this case from agricultural burning. Maximum concentrations at Natal are similar to values found at mid-latitudes in summer. Tropical ozone exhibits strong spatial and temporal variability.