Carbon monoxide (CO) is one of the main reactive trace gases in the earth's atmosphere: it influences both the atmospheric chemistry and the climate. In order to evaluate the atmospheric budget of CO, knowledge of the emission rates from, and the, geographic distribution of, the individual sources are required. CO is released into the atmosphere naturally and anthropogenically: at present, the emission rates from these two categories of sources are comparable. However, the rates of emission from man-made sources have been increasing steadily since industrialisation began, and are expected to continue growing with changes in population and economy. Although the major sources of CO are known, estimates for their emission rates are still uncertain. Experimental data are available only for a limited number of locations, and more measurements in representative regions world-wide are required in order to evaluate the CO global budget more accurately.