A comprehensive climatology of stratosphere--troposphere exchange (STE), based on 15 years of ECMWF global atmospheric reanalysis data for the period 1979--1993, has been carried out with a specially tuned version of the Lagrangian particle dispersion model, FLEXPART. The typical amplitudes, timescales, seasonality, and spatial variability of STE revealed in the climatology are described. Mean distributions of STE air masses of various ages relate to aspects of the global circulation, while anomaly patterns, such as the North Atlantic Oscillation (NAO) and El Ni¿o, influence STE variability. Characteristic differences are seen between shallow STE flows remaining within the tropopause region (TPR) and deep STE transports, which include cases of stratospheric air reaching the lower troposphere. Unlike for shallow STE events, such deep intrusions show a distinct winter maximum. This subsequently impacts on the seasonal cycle of stratospheric ozone in the troposphere, which thus depends on altitude but also on the decay timescale of ozone. A much broader view of STE than has previously been possible is enabled. The inherent scope of STE is far wider than a discussion of cross-tropopause fluxes alone. Indeed, more than 90% of all cross-tropopause flows return within 6 hours. It is thus crucial to distinguish between short-term STE, during which air parcels rapidly recross the tropopause, and deep and/or long-term STE, in which air parcels have subsequently long residence times away from their source.