Dioxins and dioxin-like compounds are a group of chemical compounds that are persistent environmental pollutants. Some of them are highly toxic, but the toxicity among them varies 30,000-fold. They are grouped together, because their mechanism of action is the same. They activate aryl hydrocarbon receptor, albeit with very different binding affinities, leading to high differences in toxicity and other effects. They are mostly minor by-products of burning or various industrial processes - or, in case of dioxin-like PCBs and PBBs, unwanted minor components of intentionally produced mixtures. Dioxins have different toxicity depending on the number and position of the chlorine atoms. Because dioxins refer to such a broad class of compounds that vary widely in toxicity, the concept of toxic equivalency factor has been developed to facilitate risk assessment and regulatory control. TEFs exist for seven congeners of dioxins, ten furans and twelve PCBs. The reference congener is the most toxic dioxin TCDD which per definition has a TEF of one. In essence, multiplying the amount of a particular congener with its TEF produces the amount toxicologically equivalent to TCDD, and after this conversion all dioxin-like congeners can be summed up, and the resulting toxicity equivalent quantity gives an approximation of toxicity of the mixture measured as TCDD.