A great variety of drugs, cosmetics, food ingredients as well as environmental contaminants are secreted with human milk as a result of actual exposure or the accumulated body burden of the mother. Of great concern and least amenable to short-term intervention are persistent substances in the environment with long half-lives in the body due to their lipophilic properties and minimal degradation. Polyhalogenated aromatic hydrocarbons, namely organochlorine pesticides, polychlorinated biphenyls (PCB) and polychlorinated dibenzodioxins (PCDD) and dibenzofurans (PCDF) are fetotoxic, neurotoxic, immunotoxic, some are promoting carcinogens and/or interfere with hormonal receptors. They pass the placenta and equilibrate among the lipid compartments of the body including breast milk lipids. Transplacental exposure is more relevant with regard to physical development and cognitive functioning of the child than postnatal exposure via breastmilk. Restrictions for production, use and release have been successful in decreasing exposure as shown by a downward trend of their contents both in human milk and serum lipids for the last 15 to 20 years. It is difficult to evaluate the potentially late effects of the exposure via breastmilk which is 10 to 100 times higher in industrialised countries than the tolerable daily intake (TDI) of 1 to 4 toxic equivalents (WHO-TEQ) pg/kg/day established in 1998 by WHO for dioxins and dioxin-like PCBs but which lasts for 0.6% of the expected life span only. Carefully conducted long-term follow-up of cohorts with defined exposure levels, with consideration of numerous biological and psychological parameters, is expected to provide the answer.