The only thing that all pesticides have in common is that they are used to control pests. Otherwise, they come from almost every imaginable class of chemicals. Everyone associated with pesticide use—farmers, Extension Services, the U.S. EPA, state regulatory agencies, manufacturers, and environmentalists—need information that will allow them to distinguish between pesticides that may be a problem as pollutants in certain situations and those that may not. There are five basic properties that, when combined with information about site and use, provide much information about the potential of a pesticide to be a pollutant. These five properties are solubility in water, volatility, soil sorption tendency, persistence, and ionization potential. We have compiled the most complete collection of these properties available, using the compilations of others but verifying values from the primary literature in many cases. The quality and completeness of these data is the result of the cooperation of many individuals and pesticide manufacturers. A complete primary literature search was not done, however.