Observations of numerical changes in the insect fauna in the field are very difficult to interpret in relation to causal factors, because natural insect populations fluctuate widely and there is serious lack of baseline data on the extent of natural changes. This chapter reviews the present knowledge and attempts to outline the general mechanisms that seem to be involved in changes in insect communities in a polluted environment. Part of the differential response between insect species may be caused by variable sensitivity to the direct effects of toxicants. Only a few insect species, however, have been investigated for tolerance to air pollutants and few of these are herbivores. A number of field studies imply that changes in parasitism and predation may play a role in the release of herbivorous insects from innocuous levels in polluted areas. Air pollution has been shown to change the biochemical composition of many plants and hence the nutritional value of the plants to herbivores.