The abundance of ozone in the stratosphere is, in part, determined by photochemical production of ozone and the abundances of reactive trace chemical species (HOx, NOx, ClOx, BrOx) that catalyse ozone removal processes. Significant stratospheric ozone losses, first detected during spring over Antarctica, now extend to all seasons and all latitudes poleward of 30° in both hemispheres. Observations have demonstrated that gasp- hase and heterogeneous chlorine and bromine chemistries play major roles in the chemical destruction of ozone in both the mid-latitude and polar lower stratospheres. Agricultural, industrial and domestic activities can influence the abundances of these ozone-destroying catalysts and therefore can affect the levels of ozone in the stratosphere. The CFCs, long-lived chlorinated solvents, the HCFCs, halons and methyl bromide are the major anthropogenic sources of stratospheric chlorine and bromine. Observations from monitoring networks worldwide have demonstrated either slowdowns or reversals in the growth rates of most of the major halocarbon species (CFCs, long-lived chlorinated solvents, halons), whereas the HCFCs, which are interim replacement chemicals for CFCs under the Montreal Protocol, are still increasing rapidly and the HFCs, the long-term replacements for CFCs, have now been found in the atmosphere for the first time. At present the Montreal Protocol appears to be working in controlling ozone-depleting substances (ODSs) in the background atmosphere. However, there may be still some problems ahead for stratospheric ozone, associated with the economic costs of replacement of some ODSs, the growth of the use of ODSs in the developing world and the uncertain effect of long-term climate change on ozone depletion.