The role of ozone in absorbing ultraviolet solar radiation is well known.Ozone also makes a significant contribution to the radiative balance of the upper troposphere and lower stratosphere, such that changes in the distribution of ozone in these atmospheric regions will affect the radiative forcing of climate [1,2]. Furthermore, tropospheric ozone is the source of the hydroxyl radical which controls the abundance and distribution of many atmospheric constituents, including greenhouse gases such as methane and hydrochlorofluorocarbons. Tropospheric ozone is produced photochemically in situ and is also transported down from the stratosphere, but the relative importance of these two sources to its global budget is poorly understood. High-quality tropospheric and lower-stratospheric ozone profile measurements are available from sondes and lidar techniques, but their geographical sampling is very limited. Complementary satellite measurements of the global ozone distribution in this height region are therefore required to quantify ozone's tropospheric budget and its participation in climate-forcing and tropospheric chemistry. Here we present direct measurements of tropospheric ozone concentrations from space, made by the European Space Agency's Global Ozone Monitoring Experiment. These results demonstrate the potential of satellite measurements to provide self-consistent tropospheric and stratospheric ozone distributions on a global scale.