Greek physician, writer, and philosopher who exercised a dominant influence on medical theory and practice in Europe from the Middle Ages until the mid-17th century. His authority in the Byzantine world and the Muslim Middle East was similarly long-lived.

The son of a wealthy architect, Galen was educated as a philosopher and man of letters. His hometown, Pergamum, was the site of a magnificent shrine of the healing god, Asclepius, that was visited by many distinguished figures of the Roman Empire for cures. When Galen was 16, he changed his career to that of medicine, which he studied at Pergamum, at Smyrna (modern İzmir, Tur.), and finally at Alexandria in Egypt, which was the greatest medical centre of the ancient world. After more than a decade of study, he returned in AD 157 to Pergamum, where he served as chief physician to the troop of gladiators maintained by the high priest of Asia.

In 162 the ambitious Galen moved to Rome. There he quickly rose in the medical profession owing to his public demonstrations of anatomy, his successes with rich and influential patients whom other doctors had pronounced incurable, his enormous learning, and the rhetorical skills he displayed in public debates. Galen's wealthy background, social contacts, and a friendship with his old philosophy teacher Eudemus (Eudemus Of Rhodes) further enhanced his reputation as a philosopher and physician.

Galen abruptly ended his sojourn in the capital in 166. Although he claimed that the intolerable envy of his colleagues prompted his return to Pergamum, an impending plague in Rome was probably a more compelling reason. In 168–169, however, he was called by the joint emperors Lucius Verus and Marcus Aurelius to accompany them on a military campaign in northern Italy. After Verus' sudden death in 169, Galen returned to Rome, where he served Marcus Aurelius and the later emperors Commodus and Septimius Severus as a physician. Galen's final works were written after 207, which suggests that his Arab biographers were correct in their claim that he died at age 87, in 216/217.

Galen regarded anatomy as the foundation of medical knowledge, and he frequently dissected and experimented on such lower animals as the Barbary ape (Barbary macaque) (or African monkey), pigs, sheep, and goats. Galen's advocacy of dissection, both to improve surgical skills and for research purposes, formed part of his self-promotion, but there is no doubt that he was an accurate observer. He distinguished seven pairs of cranial nerves, described the valves of the heart, and observed the structural differences between arteries and veins. One of his most important demonstrations was that the arteries carry blood, not air, as had been taught for 400 years. Notable also were his vivisection experiments, such as tying off the recurrent laryngeal nerve to show that the brain controls the voice, performing a series of transections of the spinal cord to establish the functions of the spinal nerves, and tying off the ureters to demonstrate kidney and bladder functions. Galen was seriously hampered by the prevailing social taboo against dissecting human corpses, however, and the inferences he made about human anatomy based on his dissections of animals often led him into errors. His anatomy of the uterus, for example, is largely that of the dog's.

Galen's physiology was a mixture of ideas taken from the philosophers Plato and Aristotle as well as from the physician Hippocrates, whom Galen revered as the fount of all medical learning. Galen viewed the body as consisting of three connected systems: the brain and nerves, which are responsible for sensation and thought; the heart and arteries, responsible for life-giving energy; and the liver and veins, responsible for nutrition and growth. According to Galen, blood is formed in the liver and is then carried by the veins to all parts of the body, where it is used up as nutriment or is transformed into flesh and other substances. A small amount of blood seeps through the lungs between the pulmonary artery and pulmonary veins, thereby becoming mixed with air, and then seeps from the right to the left ventricle of the heart through minute pores in the wall separating the two chambers. A small proportion of this blood is further refined in a network of nerves at the base of the skull (in reality found only in ungulates) and the brain to make psychic pneuma, a subtle material that is the vehicle of sensation. Galen's physiological theory proved extremely seductive, and few possessed the skills needed to challenge it in succeeding centuries.

Building on earlier Hippocratic conceptions, Galen believed that human health requires an equilibrium between the four main bodily fluids, or humours (humour)—blood, yellow bile, black bile, and phlegm. Each of the humours is built up from the four elements and displays two of the four primary qualities: hot, cold, wet, and dry. Unlike Hippocrates, Galen argued that humoral imbalances can be located in specific organs, as well as in the body as a whole. This modification of the theory allowed doctors to make more precise diagnoses and to prescribe specific remedies to restore the body's balance. As a continuation of earlier Hippocratic conceptions, Galenic physiology became a powerful influence in medicine for the next 1,400 years.

Galen was both a universal genius and a prolific writer: about 300 titles of works by him are known, of which about 150 survive wholly or in part. He was perpetually inquisitive, even in areas remote from medicine, such as linguistics, and he was an important logician who wrote major studies of scientific method. Galen was also a skilled polemicist and an incorrigible publicist of his own genius, and these traits, combined with the enormous range of his writings, help to explain his subsequent fame and influence.

Galen's writings achieved wide circulation during his lifetime, and copies of some of his works survive that were written within a generation of his death. By AD 500 his works were being taught and summarized at Alexandria, and his theories were already crowding out those of others in the medical handbooks of the Byzantine world. Greek manuscripts began to be collected and translated by enlightened Arabs in the 9th century, and in about 850 Ḥunayn ibn Isḥāq, an Arab physician at the court of Baghdad, prepared an annotated list of 129 works of Galen that he and his followers had translated from Greek into Arabic or Syriac. Learned medicine in the Arabic world thus became heavily based upon the commentary, exposition, and understanding of Galen.

Galen's influence was initially almost negligible in western Europe except for drug recipes, but from the late 11th century Ḥunayn's translations, commentaries on them by Arab physicians, and sometimes the original Greek writings themselves were translated into Latin. These Latin versions came to form the basis of medical education in the new medieval universities. From about 1490, Italian humanists felt the need to prepare new Latin versions of Galen directly from Greek manuscripts in order to free his texts from medieval preconceptions and misunderstandings. Galen's works were first printed in Greek in their entirety in 1525, and printings in Latin swiftly followed. These texts offered a different picture from that of the Middle Ages, one that emphasized Galen as a clinician, a diagnostician, and above all, an anatomist. His new followers stressed his methodical techniques of identifying and curing illness, his independent judgment, and his cautious empiricism. Galen's injunctions to investigate the body were eagerly followed, since physicians wished to repeat the experiments and observations that he had recorded. Paradoxically, this soon led to the overthrow of Galen's authority as an anatomist. In 1543 the Flemish physician Andreas Vesalius showed that Galen's anatomy of the body was more animal than human in some of its aspects, and it became clear that Galen and his medieval followers had made many errors. Galen's notions of physiology, by contrast, lasted for a further century, until the English physician William Harvey correctly explained the circulation of the blood. The renewal and then the overthrow of the Galenic tradition in the Renaissance had been an important element in the rise of modern science, however.