General anaesthesia (UK) or general anesthesia (US) is medically induced loss of consciousness that renders a patient unarousable even by painful stimuli. It is achieved through medications, which can be injected or inhaled, often with an analgesic and neuromuscular blocking agent.

General anaesthesia is usually performed in an operating theatre to allow surgical procedures that would otherwise be intolerably painful for a patient, or in an intensive care unit or emergency department to facilitate endotracheal intubation and mechanical ventilation in critically ill patients. Depending on the procedure, general anaesthesia may be optional or required. No matter whether the patient prefers to be unconscious or not, certain pain stimuli can lead to involuntary responses from the patient, such as movement or muscle contractions, that make the operation extremely difficult. Thus, for many procedures, general anaesthesia is necessary from a practical point of view.

The patient's natural breathing may be inadequate during the procedure and intervention is often necessary to protect the airway.

Various drugs are used to achieve unconsciousness, amnesia, analgesia, loss of reflexes of the autonomic nervous system, and in some cases paralysis of skeletal muscles. The best combination of anaesthetics for a given patient and procedure is chosen by an anaesthetist or other specialist in consultation with the patient and the surgeon or practitioner performing the procedure.

Attempts at producing general anaesthesia can be traced throughout recorded history in the writings of the ancient Sumerians, Babylonians, Assyrians, Egyptians, Greeks, Romans, Indians, and Chinese. During the Middle Ages, scholars made advances in the Eastern world and Europe.

The Renaissance saw advances in anatomy and surgical technique. However, surgery remained a treatment of last resort. Largely because of the associated pain, many patients chose certain death over surgery. Although there has been debate as to who deserves the most credit for the discovery of general anaesthesia, scientific discoveries in the late 18th and early 19th centuries were critical to the eventual introduction and development of modern anaesthetic techniques.

Two enormous leaps occurred in the late 19th century, which allowed the transition to modern surgery. An appreciation of the germ theory of disease led to the development of antiseptic techniques in surgery. Antisepsis, which soon gave way to asepsis, reduced the overall morbidity and mortality of surgery to a far more acceptable rate. Concurrently, significant advances in pharmacology and physiology led to the development of general anaesthesia. On 14 November 1804, Hanaoka Seishū, a Japanese surgeon, became the first person on record to perform successful surgery using general anaesthesia.

In the 20th century, general anaesthesia's safety and efficacy improved with routine tracheal intubation and other advanced airway management techniques. Advances in monitoring and new anaesthetic agents with improved pharmacokinetic and pharmacodynamic characteristics also contributed to this trend, and standardized training programs for anaesthesiologists and nurse anaesthetists emerged.