The report is an in-depth study that attempts to assemble, organize, and interpret present-day information on arsenic and its compounds, and the effects of these substances on man, animals, and plants. Emphasis is given to the effects of arsenic on man, conclusions are drawn from the evaluation of current knowledge on the subject, and recommendations are made for further research. Although arsenic is highly toxic in many of its forms, a number of factors suggest that it probably is not a general pollution problem. In fact, there are indications that it may be an essential trace element. There is some evidence that arsenicals can be mutagenic in humans. There is epidemiologic evidence that inorganic arsenic is a skin and lung carcinogen in man. Skin cancer has occurred in association with exposure to inorganic arsenic compounds in a variety of populations, including patients treated with Fowler's solution, Taiwanese exposed to arsenic in artesian well water, workers engaged in the manufacture of pesticides, and vintners using arsenic as a pesticide. Lung cancer has been observed to be associated with inhalation exposure to arsenic in copper smelters, workers in pesticide manufacturing plants, Moselle vintners, and Rhodesian gold miners. While much arsenic enters the atmosphere from the burning of coal, the concentrations are too low to be a matter of concern.