This Article, the second in a series of five, examines the meaning of “pollutant” under the Clean Water Act. Congress and EPA have defined “pollutant” to mean a list of specific substances and broad categories of materials and wastes discharged into water, e.g., “biological materials” and “chemical wastes.” The definition is broad enough to encompass virtually all substances associated with human activity that are discharged to water, regardless of whether the substances cause pollution or are produced through human endeavor. Therefore, “pollutant” is rarely a limiting element. Instead, the issues with the definition of “pollutant” primarily address whether it includes material used in common and productive activities, such as adding hatchery-raised fish (“biological material”) to trout streams or spraying pesticides to suppress disease-bearing mosquitoes (“biological material” or “chemical wastes”). EPA can easily fix these and other problems by a better regulatory definition.