Incineration of wastes seems to be one of the major sources of PCDDs and PCDFs (dioxins). Their prevalence and extreme stability in the environment, bioavailability and bioaccumulation in the biota and human adipose tissues and breast milk are of much concern. 2,3,7,8-TCDD is one of the most toxic chemicals known and has been found to have teratogenic and carcinogenic activities in animals. Exposure to TCDD can result in chloracne, general weakness, drastic weight loss, hyperpigmentation of skin, hirsutism, porphyria cutanea tarda, liver damage, changes in activities of various liver enzymatic levels, abnormal lipid metabolism, abnormalities of the endocrine and immune systems, and possible teratogenic effects in humans. Moreover, chronic bioassay data indicate that TCDD is one of the most potent carcinogens known. It promotes liver and skin carcinogeneses, and is an initiator for various target organs in rodent test systems. According to the classification system of IARC, the qualitative evidence for carcinogenicity of TCDD is considered to be 'sufficient' in animals and 'inadequate' in humans. Consequently, TCDD has been placed in IARC's 2B category. In the absence of chronic bioassay data on other PCDDs and PCDFs, several TCDD equivalent approaches have been proposed for risk assessment on other congeners or mixtures. The paper compares the various approaches.