Human ingenuity has given us a vast range of manufactured substances. There is increasing concern that many of them, but especially pesticides and plastics, contain chemical compounds that mimic hormones normally produced within the body and so interfere with their action. As hormones are produced by the endocrine glands (they include the adrenal and thyroid glands as well as the ovaries and testes), such disruptive compounds have become known recently as endocrine disrupters. Most attention is focused on ones which mimic the action of the sex hormone oestrogen, which are generated, for example, by breakdown products of PCBs and DDT. They are under suspicion of causing breast cancer, reducing sperm counts, causing early puberty in girls, reducing the intelligence of children, and of interfering with foetal development. A major problem with discovering the truth is that large doses of the disrupters have little effect, because the body detects them and brings defences to bear; the danger comes from minute quantities which slip through the body’s defences, quantities so small that they are hard to detect and even harder to experiment with.