The evidence that human cancers result in large measure from factors related to life style, working conditions, or other extrinsic variables--and are thus, in principle, preventable (Doll and Peto, 1981)--and the hypothesis that there is no threshold for the carcinogenic effects of many cancer-causing agents (Office of Science and Technology Policy, 1985) have spurred efforts to minimize human exposure to carcinogens. In pursuit of this goal, attempts have been made to identify airborne and other extrinsic carcinogens and to assess the extent to which they contribute to the occurrence of cancer. The status of such endeavors is summarized in the following, without any attempt at a comprehensive review, which would be beyond the scope of this report.