Pesticides have been widely and heavily used in agriculture in the Great Lakes Basin (approximately 93000 tons were used in 1995 alone). Herbicides account for two-thirds of the total pesticides used. Herbicide usage in seven of the Great Lakes states (Illinois, Indiana, Michigan, Minnesota, Ohio, Pennsylvania, and Wisconsin) constituted approximately 50% of the total usage in the USA. The pesticide use is concentrated in the corn- and soybean-growing areas of the southern Lake Michigan Basin and western Lake Erie Basin of the Great Lakes. Organochlorines (OC) such as DDT and dieldrin were the major pesticides used in the Great Lakes Basin prior to 1970s. The past usage of OC insecticides was large enough to cause effects on the Great Lakes ecosystem. Whereas environmental concentrations of OC pesticides in the Great Lakes Basin have generally declined during the past 20 years, concerns nevertheless remain, because these substances persist in the environment and accumulate in the food chain. There continue to be fish consumption advisories based on unacceptable levels of OC pesticides in sport and commercial fish from the Great Lakes. Atmospheric transport from agricultural regions in the USA and Canada, where these pesticides were used extensively in the past, continues to be a source of contamination in the Great Lakes. Hazardous waste sites with elevated levels of OC pesticides represent another source. In the 1980s and the 1990s, OC pesticides were replaced with new-generation pesticides, which are more target-specific and less persistent. Among herbicides, atrazine, metolachlor, cyanazine, acetolachlor, and alachlor account for about 53% of the total usage, and among insecticides, organophosphates (OP) such as malathion, chlorpyrifos, terbufos, diazinon, and methyl-parathion account for 72% of the total usage in the late 1990s. Various monitoring programs have shown that the levels of OC pesticides have declined steadily until the 1980s, and thereafter the rate of this decline has slowed. The relative slow decline or steady state in OC levels in the 1990s was thought to be due to release/re-suspension and/or recycling of these compounds through the Great Lakes ecosystem. Atmospheric deposition has become an increasingly significant route of entry of OC pesticides into the Great Lakes ecosystem, although such depositions have decreased recently, and the lakes are now acting as a source via the degassing of these compounds. Most of the current-use pesticides are not bioaccumulative; however, because of the high volume of their usage, these compounds are present in Great Lakes waters. The most frequently detected herbicides in waters include several triazines (atrazine, cyanazine, and simazine), acetanilides (metolachlor and alachlor), and 2,4-D. In addition to streams and rivers, atmospheric transport is a pathway for current-use pesticides in the Great Lakes. The occurrence of in-use pesticides in surface waters follows broad and complex patterns in land use and associated pesticide use. In general, concentrations showed an increasing gradient from north to south, with Superior < Huron < Ontario < Erie. Although most of the current-use pesticides are not bioaccumulative, exposure of aquatic organisms to these compounds can be deleterious. Current-use pesticides can undergo environmental and biological transformations, although the degradates of the most heavily used herbicides found in surface water have not been studied widely. In many cases, methods to assess the fate of current-use pesticides and their degradates in the environment are not available. Future investigations should focus on the fate and effects of current-use pesticides.