The importance of insects as vectors of disease, as pests, and as destroyers of crops has long been recognized, and the necessity for their control has led to the development of increasing numbers of new pesticidal agents. There are some inevitable hazards that accompany the use of these toxic materials. Each year a number of deaths and many more illnesses of varying degrees of severity are attributed to pesticides. Some of the newer organic phosphorus compounds, particularly parathion, and a few of the chlorinated hydrocarbon materials, such as dieldrin, have caused occupational poisoning. This paper will review some of the studies that have been done to evaluate the hazard from both occupational and environmental exposure to pesticides.