The specific mechanisms of carcinogenesis and teratogenesis are poorly understood. There are, however, some known or potential common mechanisms, such as gene or chromosome mutations, interference with gene expression, altered membrane properties, or altered intracellular homeostasis. Carcinogenesis is generally regarded as a multistage process, and a carcinogen can act at one or several stages. Agents acting in the early stages of the neoplastic process are DNA-reactive, mutagenic compounds which enable cells to be transformed to malignancy. These agents can also, if acting during critical periods of ontogenesis, induce abnormal development of the embryo. Agents which block gap junctional intercellular communication may act both as tumor-promoting agents and ans teratogens in the developing embryo. Hormones are essential in the control of development and differentiation. Modulation of the intracellular hormone receptors may lead to changes in homeostasis with abnormal cellular proliferation and development as a consequence.