Persistent and toxic chemicals such as polychlorinated biphenyls, dibenzo-p-dioxins and dibenzofurans are able to accumulate in human tissues, especially in fats. In recent years PCBs have frequently been detected in human milk, blood and adipose tissue from the general population and levels in different tissues correspond to the fat content of the samples. In recent years the very rapid development of analytical methods and the lowering of detection limits have resulted in the detection of PCDDs and PCDFs in human tissues from the general population. The levels of PCBs are approximately 10,000 times higher than the sum of the levels of PCDDs and PCDFs, but the latter substances are much more toxic. Accidental or occupational exposures have resulted in much higher tissue levels in exposed individuals than in the general population. The relatively high levels in breast milk result in very high daily intakes of such chemicals by breast-fed infants, and a potential hazard to this risk group. The origin of the residues seems mainly to be environmental pollution from waste disposal, incineration or other high-temperature operations.