I present an analysis of trends in ozone since about 1970 and discuss the quality of the ozonesonde data and inconsistencies among data records. In the troposphere there are significant spatial variations in the trends, with largest increases found over Europe (∼2% yr−1 throughout the troposphere) and no long-term trend over Canada; there is a small (<1% yr−1) increase over the east coast of the United States in summer. The Japanese stations show increases in ozone only below 5.5 km. The trends in tropospheric ozone are discussed in the context of trends in emissions of nitrogen oxides from surface sources and from aircraft. Trends in surface emissions of NOx have been similar in the United States, and Western Europe, while the trend in ozone has been larger over Europe; the cause of the large increase in ozone over Europe is unclear. The increase over Europe has leveled off in recent years, and there is no increase since 1980 over the eastern United States. The lack of an ozone increase in the last decade over these two regions during a period of rapid growth of aircraft traffic argues against a significant influence from emissions of NOx from aircraft. The large interannual variability in ozone in the upper troposphere is similar to that in the lower stratosphere. Any short-term trend in ozone near the tropopause could be caused simply by dynamical factors. Stratospheric ozone decreases are found from about 24 km to near the tropopause. Ozone losses below 17 km appear to be responsible for the 20% difference between trends in column ozone derived from the Stratospheric Aerosol and Gas Experiment (SAGE) and the total ozone mapping spectrometer (TOMS). Ozone changes in the troposphere make an important contribution to the column ozone change for some stations. The stratospheric decreases are larger in winter than in summer over Europe and the midlatitude stations of North America; they are larger in summer than in winter over the high latitude (>53°N) stations of North America. These seasonal losses are consistent with the patterns reported by Stolarski et al. (1992) using TOMS data. Losses are found year round over Syowa, Antarctica, although they are largest in spring.