Hippocrates of Cos or Hippokrates of Kos (ca. 460 BC – ca. 370 BC) was an ancient Greek physician of the Age of Pericles (known as Classical Athens), and is considered one of the most outstanding figures in the history of medicine. He is referred to as the ' Father of Western Medicine' in recognition of his lasting contributions to the field as the founder of the Hippocratic School of medicine. This intellectual school revolutionized medicine in ancient Greece, establishing it as a discipline distinct from other fields that it had traditionally been associated with (notably theurgy and philosophy), thus making medicine a profession.

However, the achievements of the writers of the Corpus, the practitioners of Hippocratic medicine, and the actions of Hippocrates himself are often confusing; thus very little is known about what Hippocrates actually thought, wrote, and did. Nevertheless, Hippocrates is commonly portrayed as the paragon of the ancient physician. In particular, he is credited with greatly advancing the systematic study of clinical medicine, summing up the medical knowledge of previous schools, and prescribing practices for physicians through the Hippocratic Oath, Corpus and other works.

Soranus of Ephesus, a 2nd-century Greek gynecologist, was Hippocrates' first biographer and is the source of most information on Hippocrates' person. Information about Hippocrates can also be found in the writings of Aristotle, which date from the 4th century BC, in the Suda of the 10th century AD, and in the works of John Tzetzes, which date from the 12th century AD.

Soranus said that Hippocrates learned medicine from his father and grandfather, and studied other subjects with Democritus and Gorgias. Hippocrates was probably trained at the asklepieion of Kos, and took lessons from the Thracian physician Herodicus of Selymbria. The only contemporaneous mention of Hippocrates is in Plato's dialogue Protagoras, where Plato describes Hippocrates as "Hippocrates of Kos, the Asclepiad". Hippocrates taught and practiced medicine throughout his life, traveling at least as far as Thessaly, Thrace, and the Sea of Marmara. He probably died in Larissa at the age of 83 or 90, though some accounts say he lived to be well over 100; several different accounts of his death exist.

He based his medical practice on observations and on the study of the human body. He is credited with being the first physician to reject superstitions, legends and beliefs that credited supernatural or divine forces with causing illness by possession of evil spirits and disfavor of the gods. He held the belief that illness had a physical and a rational explanation.

Hippocrates was credited by the disciples of Pythagoras of allying philosophy and medicine. He separated the discipline of medicine from religion, believing and arguing that disease was not a punishment inflicted by the gods but rather the product of environmental factors, diet, and living habits. Indeed there is not a single mention of a mystical illness in the entirety of the Hippocratic Corpus. However, Hippocrates did work with many convictions that were based on what is now known to be incorrect anatomy and physiology, such as Humorism.

Hippocrates held the belief that the body must be treated as a whole and not just a series of parts. He accurately described disease symptoms and was the first physician to accurately describe the symptoms of pneumonia, as well as epilepsy in children. He believed in the natural healing process of rest, a good diet, fresh air and cleanliness. He noted that there were individual differences in the severity of disease symptoms and that some individuals were better able to cope with their disease and illness than others. He was also the first physician that held the belief that thoughts, ideas, and feelings come from the brain and not the heart as others of him time believed.

Ancient Greek schools of medicine were split (into the Knidian and Koan) on how to deal with disease. The Knidian school of medicine focused on diagnosis. Medicine at the time of Hippocrates knew almost nothing of human anatomy and physiology because of the Greek taboo forbidding the dissection of humans. The Knidian school consequently failed to distinguish when one disease caused many possible series of symptoms. The Hippocratic school or Koan school achieved greater success by applying general diagnoses and passive treatments. Its focus was on patient care and prognosis, not diagnosis. It could effectively treat diseases and allowed for a great development in clinical practice.

Hippocratic medicine and its philosophy are far removed from that of modern medicine. Now, the physician focuses on specific diagnosis and specialized treatment, both of which were espoused by the Knidian school. This shift in medical thought since Hippocrates' day has caused serious criticism over the past two millennia, with the passivity of Hippocratic treatment being the subject of particularly strong denunciations; for example, the French doctor M. S. Houdart called the Hippocratic treatment a "meditation upon death".

Hippocratic medicine was humble and passive. The therapeutic approach was based on "the healing power of nature" ("vis medicatrix naturae" in Latin). According to this doctrine, the body contains within itself the power to re-balance the four humours and heal itself (physis). Hippocratic therapy focused on simply easing this natural process. To this end, Hippocrates believed that "rest and immobilization were of capital importance". In general, the Hippocratic medicine was very kind to the patient; treatment was gentle, and emphasized keeping the patient clean and sterile. For example, only clean water or wine were ever used on wounds, though "dry" treatment was preferable. Soothing balms were sometimes employed, many made from herbs and plants found locally.

One of the strengths of Hippocratic medicine was its emphasis on prognosis. At Hippocrates' time, medicinal therapy was quite immature, and often the best thing that physicians could do was to evaluate an illness and induce its likely progression based upon data collected in detailed case histories.

Hippocrates traveled throughout Greece practicing his medicine. He founded a medical school on the island of Cos, Greece and began teaching his ideas. He soon developed an Oath of Medical Ethics for physicians to follow. This Oath is still taken by physicians today as they begin their medical practice. His passive approach was very successful in treating relatively simple ailments such as broken bones which required traction to stretch the skeletal system and relieve pressure on the injured area. The Hippocratic bench and other devices were used to this end. One of the strengths of Hippocratic medicine was its emphasis on prognosis.

Hippocrates and his followers were first to describe many diseases and medical conditions. He is given credit for the first description of clubbing of the fingers, an important diagnostic sign in chronic suppurative lung disease, lung cancer and cyanotic heart disease. For this reason, clubbed fingers are sometimes referred to as "Hippocratic fingers". Hippocrates was also the first physician to describe Hippocratic face in Prognosis. Shakespeare famously alludes to this description when writing of Falstaff's death in Act II, Scene iii. of Henry V.

Hippocrates began to categorize illnesses as acute, chronic, endemic and epidemic, and use terms such as, "exacerbation, relapse, resolution, crisis, paroxysm, peak, and convalescence." Another of Hippocrates' major contributions may be found in his descriptions of the symptomatology, physical findings, surgical treatment and prognosis of thoracic empyema, i.e. suppuration of the lining of the chest cavity. His teachings remain relevant to present-day students of pulmonary medicine and surgery. Hippocrates was the first documented chest surgeon and his findings are still valid.

The Hippocratic school of medicine described well the ailments of the human rectum and the treatment thereof, despite the school's poor theory of medicine. Hemorrhoids, for instance, though believed to be caused by an excess of bile and phlegm, were treated by Hippocratic physicians in relatively advanced ways. Cautery and excision are described in the Hippocratic Corpus, in addition to the preferred methods: ligating the hemorrhoids and drying them with a hot iron. Other treatments such as applying various salves are suggested as well. Today, "treatment for hemorrhoids still includes burning, strangling, and excising". Also, some of the fundamental concepts of proctoscopy outlined in the Corpus are still in use. For example, the uses of the rectal speculum, a common medical device, this constitutes the earliest recorded reference to endoscopy.

The Hippocratic Oath, a seminal document on the ethics of medical practice, was attributed to Hippocrates in antiquity although new information shows it may have been written after his death. This is probably the most famous document of the Hippocratic Corpus. Recently the authenticity of the document's author has come under scrutiny. While the Oath is rarely used in its original form today, it serves as a foundation for other, similar oaths and laws that define good medical practice and morals. Such derivatives are regularly taken today by medical graduates about to enter medical practice.

Hippocrates died in 377 BC. His contributions revolutionized the practice of medicine; but after his death the advancement stalled. So revered was Hippocrates that his teachings were largely taken as too great to be improved upon and no significant advancements of his methods were made for a long time. The centuries after Hippocrates' death were marked as much by retrograde movement as by further advancement. For instance, "after the Hippocratic period, the practice of taking clinical case-histories died out.