Ice cores provide the most direct evidence available about the past atmosphere. For long-lived trace gases, ice cores have provided clear evidence that in the last two centuries, concentrations of several greenhouse gases have risen well outside the natural range observed in the previous 650 000 years. Major natural changes are also observed between cold and warm periods. Aerosol components have to be interpreted in terms of changing sources, transport and deposition. When this is done, they can also supply evidence about crucial aspects of the past environment, including sea ice extent, trace element deposition to the ocean, and about the aerosols available for cloud nucleation, for example. It is much more difficult to extract information about shorter-lived chemical species. Information may be available in components such as nitrate and formaldehyde, but to extract that information, detailed modern atmospheric studies about air to snow transfer, preservation in the ice, and the link between the polar region boundary layer and other parts of the atmosphere are urgently required.