Summary: Agents that raise the frequency of mutation above the spontaneous or background rate, or that incite development of a carcinoma or any other sort of malignancy. A mutagen is a substance or agent that induces heritable change in cells or organisms. A carcinogen is a substance that induces unregulated growth processes in cells or tissues of multicellular animals, leading to the disease called cancer (Fig. 1). Although mutagen and carcinogen are not synonymous terms, the ability of a substance to induce mutations and its ability to induce cancer are strongly correlated. In general, mutagenesis refers to processes that result in genetic change, and carcinogenesis (the processes of tumor development) may result from mutagenic events