This is an overview of environmental toxicology in its widest sense, which includes both the ecological and the health effects of environmental contaminants. After a brief introduction, basic concepts of ecology are shown, together with two case histories illustrating pathways by which accidental or deliberate dispersal of chemicals can imbalance ecosystems. Factors governing weather and climate are described, as are interactions between anthropogenic air pollutants and natural constituents of the atmosphere, and their role in global warming. The terrestrial effects of air pollutants in generating acid rain are dealt with as well as the health effects of air pollution, including radiation and contaminants of indoor air. Pollution of water (and, implicitly, of soil) is discussed together with the concepts of persistence and bioaccumulation and their consequences in the risks posed by specific chemicals.