A large proportion of cancers is preventable. External factors, discovered by epidemiological studies during the last 50 years, account for a majority of all cancer deaths. However, still rather little is known, about how environmental and genetic factors interact, how they may regulate gene activation etc. And it is a long way from the discovery of a basic regulatory mechanism to practical patient treatment. This volume describes the present state of the art in carcinogenesis, possibilities for cancer prevention, and gives genetic background in cancer development. Attention is given to the host-environment interaction, considering how this interaction may lead to cancer formation and how it can be utilised in cancer prevention. The molecular basis for cancer development and the molecular basis for prevention are described.