Publisher Summary This chapter discusses general aspects of pesticides, from their physicochemical properties to their relevance in the environment. The assessment of distribution and effects of pesticide residues in the environment requires a consideration of the usage of pesticides that were applied in the past and at the present. The pesticide market is an important one with an average 12% growth in dollar terms. One notable trend has been in the increased use of herbicides and the relative decrease of fungicides. A source of pollution of ground waters is the non-agricultural use of pesticides. Most of the non-point source pollution of ground- and surface waters by pesticides is of agricultural origin (e.g., non-irrigated, irrigated, or specialty crop production, pasture land, gangland) or land-disposal origin (landfills, channelization, hazardous wastes, on flow regulation). However, in recent years, particular attention has been devoted to the non-agricultural uses of pesticides—for example, on highways and railroads and on golf courses. The non-agricultural uses of pesticides accounted for 550 tons of active ingredient in 1989, in England and Wales alone, atrazine, simazine, and diuron being the three major compounds used.