Andries van Wezel (31 December 1514 – 15 October 1564), latinized as Andreas Vesalius (/vɪˈseɪliəs/), was an anatomist and physician who wrote De Humani Corporis Fabrica Libri Septem (On the fabric of the human body in seven books), which is considered one of the most influential books on human anatomy and a major advance over the long-dominant work of Galen. Vesalius is often referred to as the founder of modern human anatomy. He was born in Brussels, which was then part of the Habsburg Netherlands. He was a professor at the University of Padua (1537–1542) and later became Imperial physician at the court of Emperor Charles V.

Vesalius was born as Andries van Wesel to his father Anders van Wesel and mother Isabel Crabbe on 31 December 1514 in Brussels, which was then part of the Habsburg Netherlands. His great-grandfather, Jan van Wesel, probably born in Wesel, received a medical degree from the University of Pavia and taught medicine at the University of Leuven. His grandfather, Everard van Wesel, was the Royal Physician of Emperor Maximilian, whilst his father, Anders van Wesel, served as apothecary to Maximilian and later valet de chambre to his successor, Charles V. Anders encouraged his son to continue in the family tradition and enrolled him in the Brethren of the Common Life in Brussels to learn Greek and Latin prior to learning medicine, according to standards of the era.

In 1528 Vesalius entered the University of Leuven (Pedagogium Castrense) taking arts, but when his father was appointed as the Valet de Chambre in 1532 he decided instead to pursue a career in medicine at the University of Paris, where he moved in 1533. There he studied the theories of Galen under the auspices of Johann Winter von Andernach, Jacques Dubois (Jacobus Sylvius) and Jean Fernel. It was during that time that he developed an interest in anatomy and was often found examining excavated bones in the charnel houses at the Cemetery of the Innocents. He is said to have constructed his first skeleton by stealing from a gibbet.

Vesalius was forced to leave Paris in 1536 owing to the opening of hostilities between the Holy Roman Empire and France and returned to the University of Leuven. He completed his studies there and graduated the following year. His doctoral thesis, Paraphrasis in nonum librum Rhazae medici Arabis clarissimi ad regem Almansorem, de affectuum singularum corporis partium curatione, was a commentary on the ninth book of Rhazes.

On the day of his graduation he was immediately offered the chair of surgery and anatomy (explicator chirurgiae) at the University of Padua. He also guest-lectured at the University of Bologna and the University of Pisa. Prior to taking up his position in Padua, Vesalius traveled through Italy and assisted the future Pope Paul IV and Ignatius of Loyola to heal those afflicted by leprosy. In Venice he met the illustrator Johan van Calcar, a student of Titian. It was with van Calcar that Vesalius published his first anatomical text, Tabulae Anatomicae Sex, in 1538. Previously these topics had been taught primarily from reading classical texts, mainly Galen, followed by an animal dissection by a barber–surgeon whose work was directed by the lecturer. No attempt was made to confirm Galen's claims, which were considered unassailable. Vesalius, in contrast, performed dissection as the primary teaching tool, handling the actual work himself and urging students to perform dissection themselves. He considered hands-on direct observation to be the only reliable resource.

Vesalius created detailed illustrations of anatomy for students in the form of six large woodcut posters. When he found that some of them were being widely copied, he published them all in 1538 under the title Tabulae anatomicae sex. He followed this in 1539 with an updated version of Winter's anatomical handbook, Institutiones anatomicae.

In 1539 he also published his Venesection Epistle on bloodletting. This was a popular treatment for almost any illness, but there was some debate about where to take the blood from. The classical Greek procedure, advocated by Galen, was to collect blood from a site near the location of the illness. However the Muslim and medieval practice was to draw a smaller amount of blood from a distant location. Vesalius' pamphlet generally supported Galen's view but with qualifications that rejected the infiltration of Galen.

In Bologna, Vesalius discovered that all of Galen's research was restricted to animals, since the tradition of Rome did not allow dissection of the human body. Galen had dissected Barbary macaques instead, which he considered structurally closest to man. Even though Galen was a qualified examiner, his research produced many errors owing to the limited anatomical material available to him. Vesalius contributed to the new Giunta edition of Galen's collected works and began to write his own anatomical text based on his own research. Until Vesalius pointed out Galen's substitution of animal for human anatomy, it had gone unnoticed and had long been the basis of studying human anatomy.