The study of occupationally related cancers has played an important role in the control of cancer in general. The very nature of the workplace allows for studies that can help establish a relationship between an environmental exposure and cancer. Once carcinogens are identified, strategies can be developed to prevent disease. The article reviews procedures for identifying, evaluating, and categorizing human carcinogens; provides an overview of the basic epidemiologic strategies that have been used to study cancer and the workplace; and outlines the clinical evaluation of occupational exposures.