The term post-marketing surveillance is a term which should be applied to all forms of monitoring marketed drugs for all aspects of their potential to produce adverse reactions. Post-marketing surveillance to identify dangers arising from the use of medicinal products is undertaken to detect both pharmaceutical and medical hazards. Pharmaceutical hazards are sought by sampling products for evidence of contamination, mislabelling and loss of potency. Four methods currently used for the detection of medical hazards are:--(1) Quality tests, performed on batches of new products which cannot be fully evaluated by physical or chemical means (eg vaccines and blood products). (2) The "yellow card system", in which the Committee on Safety of Medicines asks doctors spontaneously to report all suspected adverse reactions to drugs. (3) Monitoring the release of a new drug, which is subject to certain restrictions that oblige doctors to feed back information to the regulatory authority. (4) Literature screening, which must be regarded as important in any comprehensive scheme of post-marketing surveillance. In addition two further methods of surveillance are described under (5) and (6):--(5) Local investigations such as intensive monitoring studies on patients receiving a particular treatment in a district general hospital or in its catchment area have been conducted in a number of centres and are cheap and successful and generate local interest and co-operation. (6) Various schemes have been proposed for national recorded release or event monitoring in which extensive studies of the clinical progress of many patients receiving the new drug are monitored.