Except in the recent geological past, palaeoatmospheric composition cannot be measured but only inferred from palaeontological or geochemical observations. The likely controls on atmospheric composition are reviewed and the role of organisms stressed as monitors, recorders, motors and moderators of palaeoatmospheric changes. Whereas there is a good general case for changes in PO2 and PCO2 during. Phanerozoic time, proposals for timing and mechanism in the earlier Phanerozoic are speculative at present. It seems probable that the late Palaeozoic spread of land plants raised O2, and reduced CO2 levels in the atmosphere. The end-Cretaceous crisis had a transient impact on the atmosphere, whose general constancy through geological time is one of the most remarkable features of Earth history.