Evidence for the direct effects of air pollutants on forests consists of field studies mainly based around point sources of pollution and numerous experimental studies where seedlings or young trees have been exposed to known concentrations of specific gaseous pollutants, pollutant combinations and/or to artificially applied acidic deposition. However, with the exception of ground level emissions of phytotoxic pollutants (e.g. HF, SO₂) in the vicinity of point sources and the mounting evidence for foliar induced injuries due to tropospheric ozone exposures in California, around Mexico City and within several specific regions of the eastern United States, the evidence for large-scale impacts of pollution on forest growth and productivity remains undetermined at this time. Major difficulties exist in the extrapolation of results obtained from short-term experimental studies with seedlings to mature trees, or even seedlings and saplings, located within forest ecosystems; similarly the relationships between the observed ozone induced foliar injuries (and pre-visual physiological perturbations) and the potential for forest species and stand level productivity and growth impacts need to be more carefully researched. Despite the difficulties, there is increasing political pressure on forest scientists to indicate at what level pollution will cause significant (growth and productivity and/or biodiversity) problems within our current and future forest situations.