The patient who feels pain can best assess the degree of its relief, and therefore the effective pharmacological treatment of pain requires the active participation of the patient, Patient-controlled analgesia (PCA) involves the patient in a direct manner; the patient is given control over at least some aspects (usually the timing of doses) of analgesic drug administration. A wide range of therapeutic strategies fall under the umbrella of PCA, In one major area of application, the control of severe acute pain such as postoperative pain, the term PCA is used to describe a specific type of treatment, This involves a device consisting of a pump, containing a reservoir of drug, and a handset that administers a dose of drug when activated by the patient, Such a system was first used by Sechzer over 20 years ago, Today, the device may be a microprocessor-controlled system, able to implement complex instructions programmed by the prescriber and keep a record of the patient-device interactions, or it may be a simple disposable pump powered by mechanical means such as a spring or an elastomeric drug reservoir. The usual route of PCA drug administration after surgery is intravenous, but others, particularly the epidural route, are also used. The most commonly used drugs are opioid μ-receptor agonists such as morphine and fentanyl. While many such agents have been used, the differences between them in terms of efficacy and adverse effects are modest. When administering analgesics by PCA the prescriber can choose the drug to be used and the size of each dose to be administered, and can impose certain constraints on drug administration. The most common constraint is a lockout interval, a period following the administration of a dose during which the device will not administer a second dose, even though the patient activates the handset. Within the constraints, the patient controls when doses of the drug are administered. Variants of PCA include the addition of a background infusion of the drug, which may vary or be constant, providing the patient with control over the size and timing of doses, and the addition of other drugs, such as antiemetics, to the drug reservoir. In practice, a standard PCA prescription will be used at a particular hospital (e.g. drug: morphine sulphate, demand dose: 1 mg, lockout interval: 5 minutes, background infusion: none). Although PCA is well accepted by most patients, simply initiating PCA does not guarantee good pain control. Patients must be selected appropriately, and educated preoperatively in the effective use of PCA. This information should be reinforced by nursing staff during the postoperative period. Pain control and adverse effects need to be monitored regularly, and the prescriber must be prepared to change the prescription (e.g. dose size or lockout interval) to meet the individual needs of the patient. Extensive experience with intravenous PCA has shown the risk of serious adverse events to be low. The risk is increased measurably when the patient's degree of control over drug administration is reduced, such as when a background infusion is added, and additional monitoring may then be required to maintain the level of safety. Experience with epidural PCA is more limited but, whilst there are additional risks, it is still relatively well tolerated. Although modern PCA devices are very reliable, there are a number of reports every year of patients put at risk by mistakes made by staff when initiating PCA. The direct cost of providing postoperative analgesia by PCA is greater than that of traditional approaches such as nurse-administered intramuscular injection of analgesics. However, given the good efficacy and excellent patient satisfaction, the benefits of PCA outweigh the small additional costs.