The mechanism of carcinogenesis is not yet understood. There is increasing evidence justifying the assumption that an unifying concept of carcinogenesis should be possible at the molecular level. New insights into the molecular mechanism of carcinogenesis were mainly obtained in studies on both chemical and viral carcinogenesis. In the present paper, selected results of these studies are reviewed. It is concluded that the interaction of different carcinogenic agents with the cellular DNA results in alterations of DNA. Only some of these alterations, however, seem to be relevant to carcinogenesis. Alterations of DNA can be caused by reaction of electrophilic agents with DNA constituents, by increased infidelity of DNA replication, by integration of viral genomes or by recombination events involving integrated proviruses.