Patient-controlled analgesia has successfully made the transition from research tool to clinical acceptability. Reliable and sophisticated patient-controlled analgesia systems are commercially available. The technique has been most used for control of postoperative pain but has been successfully used during labour, after burns and other trauma and in terminal care. Virtually every opioid has been administered by patient-controlled analgesia using almost every route of administration. It is more effective than the traditional techniques of pain control after surgery but is not automatically so. Choice of opioid and the settings chosen for demand dose and lockout interval greatly influence effectiveness. Patient-controlled analgesia requires active participation by the patient but the psychology of patient-controlled analgesia has generally been under-estimated. Patient-controlled analgesia has developed empirically and many assumptions have been made; there is a need for fundamental research.