Ozone plays a central role in stratospheric chemistry and dynamics. It is formed by reactions which occur following absorption of sunlight by O2 at wavelengths below 240 nm. Most atmospheric calculations and predictions have been made using simple one dimensional model. Transport is parameterized as a diffusive process and solar radiation is averaged over a range of zenith angles. There are indications that the composition of the stratosphere may vary appreciably over very short spatial scales. According to present ideas air enters the stratosphere in the tropics, in the rising part of the Hadley circulation driven by direct absorption of solar radiation. It returns to the troposphere at mid-latitudes in the vicinity of the so-called tropospheric gaps. In this picture air entering the stratosphere is freeze-dried to a mixing ratio of about 3 ppmv by passage through the tropical tropopause maintained at a temperature of about 190K.