Epidemiological observations indicate that cancers affecting different organs and systems in humans have different causes. At the descriptive level, cancer incidence and mortality rates exhibit patterns of geographic and temporal variation which are distinct and separate for each cancer site and even, at a given site, for different histological types (for instance, increasing squamous cell carcinoma of the lung and decreasing stomach cancer in most developed countries in recent decades). The existence of these distinct patterns in itself indicates that different causes are at the origin of cancers at different sites. Hence, it is of scientific and practical importance not only to identify agents that are carcinogenic to humans but also to specify as definitely as possible the target organ(s) of their action. This is done in the present review of results in the International Agency for Research on Cancer Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Humans.