In-vitro methods used for identifying and assessing tumor promoting and cocarcinogenic agents are reviewed. The distinction between tumor promotion and cocarcinogens is explained. Properties of initiators, promoters, and cocarcinogens are compared. Markers of tumor formation, resulting from exposures to tumor promoters or cocarcinogens, are discussed. In-vivo and in-vitro methods for screening chemicals for carcinogenicity are considered. Advantages of in-vitro methods over in-vivo methods are listed. These include sensitivity arising from homogeneity of the cells, rapidity, ease of quantification, economy, ability to study the response of human tissues, and the capability of studying biological events under controlled conditions with fewer variables than with in-vivo testing. Disadvantages of in-vitro systems are described. The most significant difficulty is duplicating in-vitro the reactions that occur in-vivo. In-vitro and in-vivo screening methodologies for skin, liver, respiratory tract, bladder, gastrointestinal tract, and mammary gland carcinogenesis are reviewed. Specific cell lines that are in standard use in chemical carcinogenesis studies are discussed. Promising state of the art screening systems for identifying and assessing tumor promoters and cocarcinogens are described.