Air pollution has been known to affect forests for over a century, and many of the mechanisms of pollutant deposition and effects have been established, at least for forest trees. Changes in air quality as a result of emission controls in Europe and North America, or as a result of rapid industrialisation in southern and eastern Asia, have highlighted new pollution problems. This paper, by reference to recent publications, highlights two areas where more research is required: the interactions of photochemical oxidants with biogenic emissions of volatile organic compounds, and their impact on ecological signalling; and the role of atmospheric particles in changing the leaf surface environments in forests.