In their effect on plants, air pollutants are interacting with other environmental abiotic 1 and biotic stress factors in a complex way. In particular, host plant-parasite relation- ships can be influenced considerably. In the context of forest damages and yield losses due to air pollution parasites are often involved. Air pollution as an environmental stress factor affecting host-parasite relationships has previously been discussed by several authors (Heagle, 1973; Treshow, 1975; Laurence, 1981; Hughes and Laurence, 1984: Riemer and Whittaker, 1989; Heliovaara and Vaisanen, 1993). Pollutants can act either directly or indirectly. They can be toxic to the parasite or alter its behaviour or its metabolism. Indirect effects comprise shifts in the abundance of the food plant and changes in plant surfaces, as well as variations in the host plant quality. Pollutants can also lead to alterations at higher trophic levels by affecting predators, parasitoids and pathogens (Riemer and Whittaker, 1989). Although there have been many field surveys and experimental studies carried out over the years, the complex mechanisms of changed host plant-parasite interactions under pollution stress are still not well understood. The current knowledge on changes in plant-parasite interactions in the presence of air pollutants has mainly been gained from investigations on the effects of fluorides, SO2, NOx and to a lesser extent of O3. Up to now, the majority of field studies have been carried out as gradient investigations around local sources of industrial pollutants, particularly SO2, NOx and fluorides. However, in Northern/Central Europe and North America the significance of SO2 as the most important pollutant has continuously decreased as a consequence of successful abatement strategies (Shannon, 1999; Tarrass6n, 1998). On the other hand, since the 1950s nitrogen deposition in Europe has generally increased (Goulding and Blake, 1993) (Figure 20.1). Actual nitrogen loads in Western/-Central Europe exceed the critical loads for nitrogen for various ecosystems such as forests, heathlands, mesotrophic fens, ombrotrophic bogs, species-rich grassland (UN/ECE, 1996; Posch et al., 1997). An exceedance of critical loads for N according to UN/ECE (1996) may result in shifts in plant communities towards eutrophication (Bobbink et al., 1992; Lee and Caporn, 1998) and nutrient imbalances such as increasing N and decreasing P concentrations (Fltickiger and Braun. 1998). (See also Chapter 12.) Insect herbivores are considered to be limited frequently by the availability of nitrogen. Increased nitrogen in the foliage and nutrient imbalances, respectively, may lead to enhanced susceptibility of trees to parasites. The significance of reduced and oxidized nitrogenous compounds for soil acidification is increasing due to decreasing importance of sulphur compounds. There has been conspicuous acidification of forest soils over the past decades (Falkengren-Grerup, 1987; Berggren a al., 1992; Fitze a al., 1991). Furthermore, nitrogen oxide emissions contribute to the formation of tropospheric ozone. Current ozone concentrations in Europe represent a widespread risk for forest trees and may influence insect performance. Nevertheless, field studies on ozone effects on host plants and herbivores are still rare. Until recently, there has been little interest in the impacts of elevated CO2 as the most important 'greenhouse gas' on plant-parasite relationships. It is thought that the productivity of plants may grow under elevated C02, but the quality of the host plant may decline because of an increase of carbon-based allelochemicals and a decrease in foliar nitrogen concentration (Webber et al., 1994; Hattenschwiler and Korner, 1996; Stiling et al., 1999).