The toxicity of 2,3,7,8-tetrachlorodibenzodioxin (TCDD) has been known since 1950s. TCDD is a by-product of herbicide 2,4-dichloroacetophenol (2,4-D) and 2,4,5-trichloroacetophenol (2,4,5-T), but it was first found in fryash of municipal incinerator in 1979 in Japan. In 1998, the survey of municipal incinerators revealed that 105 out of 1,641 produced above the allowed emission level of 80 ng TEQ/m3. Total annual release of dioxins is estimated to be about 5,000 g TEQ in 1997 in Japan. Japanese government started a comprehensive survey for dioxin levels in milk and blood of residents around incinerators, and their health effects. Human effects by dioxin exposures in Western countries were mostly acute and at high level in accidentally and/or occupationally. Health effects of low-dose and long lasting exposure has not been well understood. Certain amount of polychlorinated dibenzo-p-dioxins (PCDD), dibenzofurans (PCDF) and polychlorinated biphenyls (PCB) is accumulated in our body. Mother's milk is also contaminated by PCDD/PCDF. Health effects of the polychlorinated chemicals are summarized, and the necessity of regulations and recommendations for making a guideline is discussed in this review.