As noted in Chapter 1, the incidence of cancer at various tissue sites in humans varies greatly among countries and even within certain countries. Immigrants and especially their descendants tend to acquire the cancer incidences characteristic of their new habitats. The conclusion has been drawn that a high percentage, perhaps as much as 80%, of the more frequent and statisti- cally important human neoplasms (of the bronchi, stomach, colon, breast, and others) have envi- ronmental factors, including lifestyle, as major components of their etiology. This has further led to a general agreement that at least 50% of all human cancers could be avoided if existing etio- logical knowledge were applied (cf. Tomatis et al., 1997). Differences in the exposure to carci- nogenic radiations [other than solar ultraviolet (UV) light as the major cause of skin cancer], infectious disease, or hormonal factors do not appear sufficient to explain the geographical dif- ferences noted for most of the major cancers. Therefore a number of environmental chemicals, both synthetic and of natural occurrence, are under strong suspicion as carcinogens important in the etiology of much cancer in the human. Most notable among these are the chemicals in to- bacco smoke. Asbestos and certain industrial chemicals have been implicated in some human cancers. However, contrary to some reports, present cancer incidence trends do not suggest a significantly rising age-adjusted incidence if lung cancer is factored out. Other than the continu- ing rise in bronchogenic carcinoma (resulting primarily from cigarette smoking) and the sharp decline in primary gastric carcinoma, little change has occurred in the incidences of major hu- man neoplasms within the past 20 years. Suggestions that within the next decade as many as 25% of all cancers will result from occupational exposure to environmental agents have not been substantiated epidemiologically. These claims are based largely on a variety of theoretical pro- jections. Although it is important to determine the most probable causes of cancer in humans with an aim of preventing this disease, it is very important to ensure that proposed preventive measures are based on scientific fact. We must not let the fear of cancer infect our society with a cancer of fear.