A pesticide is any substance or mixture of substances intended for preventing, destroying, or controlling any pest including vectors of human or animal diseases, unwanted species of plants or animals causing harm during, or otherwise interfering with, the production, processing, storage, or marketing of food, agricultural commodities, wood and wood products, or animal feedstuffs, or which may be administered to animals for the control of insects, arachnids or other pests in or on their bodies. The physical-chemical properties usually provided for regulatory purposes are necessary to identify and describe the active ingredient and the product. Vapour pressure, water solubility, and octanol/water partition coefficient have particular value in the prediction of the environmental behavior of a pesticide. The nature and amount of data required for pesticide registration depends on the properties and use of each substance. Research resources should be focused on the identification and evaluation of major risks, and data requirements which are excessive and stifle innovation must be avoided.