Recent studies which lead to a better understanding of chemical carcinogenesis are reviewed. Almost all carcinogens require activation by metabolic processing to operate in vivo or in vitro. Some chemicals and drugs induce enzymes which metabolize carcinogens. This induction seems to be of fundamental importance to the development of human cancer resulting from environmental pollution. Nearly all lesions in cellular DNA caused by chemical carcinogens can be repaired by a DNA repair system. Failure to repair some lesions or the action of error prone repair can lead to mutations. The mutagenic effect of some chemicals provides a rapid screening test for the detection of their carcinogenic activity. Such tests are described and analyzed in detail. A general and overall scheme for possible mechanisms of action of chemical carcinogens is proposed.