We have applied a global three-dimensional chemistry-transport model to quantify the photochemistry of tropospheric O3 and compare the main source categories. We simulated a 15 year period (1979--1993) on the basis of the European Center for Medium-Range Weather Forecasts meteorological reanalyses and a time-varying emission data set. We calculate that stratosphere-troposphere exchange (STE) strongly contributes to O3 in regions where the photochemistry is quiescent. Since such regions play a minor role in radiative and chemical processes, we argue that STE-derived O3 is much less important than is suggested by its column abundance. By distinguishing between photochemical pathways in the model we calculate that tropospheric O3 in the extratropical Northern Hemisphere is strongly affected by industrial and fossil fuel-related emissions. In the tropics and Southern Hemisphere, natural emissions still play a major role. Our model results indicate a less important role for man-made biomass burning emissions than previous analyses. Further, the results show that tropospheric O3 trends are strongly influenced by transports of pollution and by meteorological variability. Scenario calculations for the year 2025 suggest that man-made emissions at low northern latitudes, in particular in southern and eastern Asia, will become a very strong tropospheric O3 source in the next decades. This will influence O3 levels on a hemispheric scale so that despite pollution regulations in Europe and North America, surface O3 will continue to grow. ¿ 2000 American Geophysical Union