John Pyle has made major contributions to our understanding of the chemistry of the stratosphere and the troposphere, both by numerical modelling of the atmosphere and by interpretation of atmospheric measurements. In the 1970s, he developed the first interactive two-dimensional model of stratospheric chemistry and transport. This enabled him to be the first person to demonstrate that ozone depletion by CFCs varies with latitude. He pioneered the analysis of satellite data for dynamical and chemical studies, for instance by testing chemical steady state relationships there and elucidating the role of the semi-annual oscillation in controlling the abundance of trace species. Since the mid-1990s, he has been in the forefront of the development of three-dimensional models incorporating chemical and transport properties. His group is the largest in Europe and a world leader scientifically. This work has demonstrated unequivocally that chemical destruction of ozone in the Arctic spring has increased during the 1990s and has shed new light on the processes which destroy ozone in mid latitudes.