In Chapter 3 the experimental basis for the induction of cancer by chemicals of both exogenous and endogenous origin was considered. In a general sense, our knowledge of chemical carcino- genesis in the human can be traced to the observation by Ramazzini (cf. Wright, 1964) of the relatively high incidence of breast cancer in Catholic nuns (Chapter 1). Ramazzini proposed that breast cancer in this occupational group was the result of their lifestyle, and today there is good evidence to argue that endogenous hormonal interactions play a dominant role in the incidence of breast cancer, especially as related to the time of childbearing (Henderson et al., 1982). The first evidence for an exogenous chemical cause of cancer was described by Hill, who related the use of tobacco snuff to the occurrence of nasal polyps (Hill, 1761). Somewhat later, Pott demonstrated the causal relationship of soot to scrotal cancer in chimney sweeps (Chapter 3). Within the last century, a number of specific chemicals, industrial processes, and physiological conditions have been shown to be causally related to increased incidences of specific human can- cers. This chapter focuses on the majority of such agents as listed by the IARC (Vainio et al., 1991) and reviewed by Doll and Peto (1981). For the sake of discussion we have divided these agents into those associated with lifestyle, with occupations, and with medical therapy and diagnosis.