The Report on Carcinogens (RoC) is a scientific public health document on cancer hazards to which people in the U.S. may be exposed. Published biennially, each edition of the report is cumulative and consists of substances newly reviewed in addition to those listed in previous editions. The Twelfth RoC contains 240 substance profiles, some of which (e.g., Estrogens, Steroidal) consist of a class of structurally related chemicals or agents. These include 54 profiles for substances listed as known to be human carcinogens and 186 profiles for substances listed as reasonably anticipated to be human carcinogens. Each profile contains: (1) the listing status, (2) a brief description of each substance, which can be chemicals, substances, mixtures, or exposure circumstances, with a summary of the evidence for its ability to cause cancer including cancer studies in humans and in experimental animals, and studies on mechanisms of carcinogenesis, which may include metabolism and genotoxicity (ability to damage genes) studies, (3) relevant information on properties, use, production, and exposure, (4) a summary of the regulations and guidelines that are likely to decrease exposure to the substance, and (5) references to scientific literature used to support the listings. The substances listed in the RoC do not include all human carcinogens.