One of the most profound topics for experimental and theoretical research into carcinogenesis and other human diseases, has been given the term gene-environment interaction (GEI). Of course one of the themes that has been repeatedly stressed in this book is the fact that environmental and genetic factors frequently interact and that disease occurrence is usually a product of such interactions. But it is one thing to say that both genes and environmental agents are important as causative agents, and another to say that such factors must undergo some sort of definitive interactive process in the causative mechanisms. Once we accept that gene environment interaction takes place, and that this interaction may often be more than simply the simultaneous presence of different risk factors, than we must address the question of how these interactions occur, both qualitatively and quantitatively.