Cancer from exposure to chemicals is known for more than two centuries. Today, approximately 40 compounds have been identified as unequivocally carcinogenic in humans, more than 300 have been shown to be carcinogenic in animal experimentation. Accordingly, an old system subdivides carcinogens as human carcinogens (A1), animal carcinogens (A2, and compounds being suspective of exerting carcinogenic activity. There exist no threshoulds of effect for notorious carcinogens. In order to improve the protection of those exposed to carcinogens in the working area, a special type of tolerance values has been introduced (technical guidance values, TRK). Contrary to MAK-values, these TRKs take into account a certain residual cancer risk which in most cases can not be quantified. The amount of acceptable residual risks is a matter of political consensus which has to be organized between the societal groups involved. For the purpose of quantitative comparisons, "unit risks" have been introduced; the problematics of this category is discussed to some extend.