I present an analysis of ozonesonde data, synthesizing what is known about the distribution of tropospheric ozone. Major features of the distribution are highlighted, and recommendations are given for testing three-dimensional models of tropospheric chemistry and transport with these data. The data are analyzed on pressure surfaces and relative to the height of the thermal tropopause. A minimum of 20 soundings are required for 95% confidence intervals of the ozone monthly means to be less than ¿30% near the extratropical tropopause. Twenty soundings also ensures means reliable to better than ¿15% for 800--500 hPa for the extratropics and for 800-100 hPa in the tropics. Ozone variability is higher in the upper troposphere for subtropical locations than for tropical locations, and 35 soundings are required for 400-100 hPa for the means to be defined to better than ¿15%. For northern middle and high latitudes, the broad summer maximum in ozone in the middle troposphere extends all the way up to the tropopause. Median concentrations at the tropopause are highest in June and July, typically 125--200 ppb, and are a factor of 2 smaller in winter. Highest values of ozone are in spring 2 km above the tropopause. The change in the phase of the annual cycle of ozone between the tropopause and the region immediately above it, and the steep concentration gradients across the tropopause, suggest that high vertical resolution (~ 1 km) will be required in models to simulate this behavior. Mean ozone values in the middle troposphere are approximately constant from 30¿ to 75¿ in the winter in both hemispheres, while there is a maximum from 35¿ to 50 ¿N in summer. In the northern subtropics, there is a summer minimum in middle tropospheric ozone over the Pacific and a summer maximum over the Atlantic which appear to be related to differences in circulation. Mean ozone values over Samoa are similar to those measured 20--30 years ago over Panama. Ozone is higher over the tropical South Atlantic (Natal) than over the western Pacific (Samoa) all year from about 800 hPa to the tropopause; ozone is most similar in May and June over the Atlantic and Pacific, the months with minimum burning in the tropics. The ozone maximum at Samoa in the middle and upper troposphere in October is caused by long-range transport of ozone and its precursors from biomass burning, with the peak lagging that at Natal by about a month. The secondary peak in ozone in January and December at South Atlantic sites reflects transport of biomass burning effluents from the Northern Hemisphere. The sonde data were used in combination with surface and satellite data to derive a gridded climatology for tropospheric ozone. ¿ 1999 American Geophysical Union