Carcinogenesis is a complex and dynamic interaction of host and environment. Important host factors include genetic constitution and health status. Among environmental factors, diet, pollutants in air and water, occupation, and life-style characteristics, such as smoking, are known to affect cancer development in the human population. This perspective represents a synthesis of the accumulated knowledge from epidemiology and animal experiments. Cancer epidemiology, in contrast to animal toxicological studies, requires no transference of the conclusions from the analysis of the animal data to indicate human carcinogenic potential. The long latency between the inception of cancer and its manifestation, and comparatively low incidence in human beings, as well as the difficulty of ascertaining exposure, have made an epidemiological approach demonstrating a cause-and-effect relationship between the suspected carcinogen and a specific cancer a very difficult if not impossible task. The recent flurry of activity in basic research in oncogenes also reinforces Knudson and others observations regarding genetic influences on cancer development.