Bloodletting
Bloodletting (or blood-letting, in modern medicine referred to as phlebotomy) was a popular medical practice from antiquity up to the late 19th century, involving the withdrawal of often considerable quantities of blood from a patient in the hopeful belief that this would cure or prevent a great many illnesses and diseases.  The practice, of unproven efficacy, has been abandoned for all except a few specific conditions as modern treatments proved or believed to be effective have been introduced. It is conceivable that historically, in the absence of other treatments for hypertension, bloodletting could sometimes have had a beneficial effect in temporarily reducing blood pressure by a reduction in blood volume.
Today the term "phlebotomy" refers to the drawing of blood for laboratory analysis or blood transfusion (see Phlebotomy (modern)). Therapeutic phlebotomy refers to the drawing of a unit of blood in specific cases like hemochromatosis, polycythemia vera, porphyria cutanea tarda etc., to reduce the amount of red blood cells.

# In the ancient world
Bloodletting is one of the oldest medical practices, having been practiced among diverse ancient peoples, including the Mesopotamians, the Egyptians, the Greeks, the Mayans, and the Aztecs. In Greece, bloodletting was in use around the time of Hippocrates, who mentions bloodletting but in general relied on dietary techniques. Erasistratus, however, theorized that many diseases were caused by plethoras, or overabundances, in the blood, and advised that these plethoras be treated, initially, by exercise, sweating, reduced food intake, and vomiting.  Herophilus advocated bloodletting. Archagathus, one of the first Greek physicians to practice in Rome, practiced bloodletting extensively and gained a most sanguinary reputation.
The popularity of bloodletting in Greece was reinforced by the ideas of Galen, after he discovered the veins and arteries were filled with blood, not air as was commonly believed at the time. There were two key concepts in his system of bloodletting. The first was that blood was created and then used up, it did not circulate and so it could 'stagnate' in the extremities. The second was that humoral balance was the basis of illness or health, the four humours being blood, phlegm, black bile, and yellow bile, relating to the four Greek classical elements of air, water, earth and fire. Galen believed that blood was the dominant humour and the one in most need of control. In order to balance the humours, a physician would either remove 'excess' blood (plethora) from the patient or give them an emetic to induce vomiting, or a diuretic to induce urination.
Galen created a complex system of how much blood should be removed based on the patient's age, constitution, the season, the weather and the place. Symptoms of plethora were believed to include fever, apoplexy, and headache. The blood to be let was of a specific nature determined by the disease: either arterial or venous, and distant or close to the area of the body affected. He linked different blood vessels with different organs, according to their supposed drainage. For example, the vein in the right hand would be let for liver problems and the vein in the left hand for problems with the spleen. The more severe the disease, the more blood would be let.  Fevers required copious amounts of bloodletting.
The Talmud recommended a specific day of the week and days of the month for bloodletting, and similar rules, though less codified, can be found among Christian writings advising which saints' days were favourable for bloodletting.  Islamic authors too advised bloodletting, particularly for fevers. The practice was probably passed to them by the Greeks; when Islamic theories became known in the Latin-speaking countries of Europe, bloodletting became more widespread. Together with cautery it was central to Arabic surgery; the key texts Kitab al-Qanum and especially Al-Tasrif li-man 'ajaza 'an al-ta'lif both recommended it. It was also known in Ayurvedic medicine, described in the Susruta Samhita.

# In the 2nd millennium
Even after the humoral system fell into disuse, the practice was continued by surgeons and barber-surgeons. Though the bloodletting was often recommended by physicians, it was carried out by barbers. This division of labour led to the distinction between physicians and surgeons. The red-and-white-striped pole of the barbershop, still in use today, is derived from this practice: the red represents the blood being drawn, the white represents the tourniquet used, and the pole itself represents the stick squeezed in the patient's hand to dilate the veins. Bloodletting was used to 'treat' a wide range of diseases, becoming a standard treatment for almost every ailment, and was practiced prophylactically as well as therapeutically.
The practice continued throughout the Middle Ages but began to be questioned in the 16th century, particularly in northern Europe and the Netherlands. In France, the court and university physicians advocated frequent phlebotomy. In England, the efficacy of bloodletting was hotly debated, declining throughout the 18th century, and briefly revived for treating tropical fevers in the 19th century.
At right are three photos and a diagram of a 19th century bloodletting device called a scarificator. It has a spring loaded mechanism with gears that snaps the blades out through slits in the front cover and back in, in a circular motion.  The case is cast brass and the mechanism and blades steel.  One knife bar gear has slipped teeth, turning the blades in a different direction than those on the other bars.  The last photo and the diagram show the depth adjustment bar at the back and sides.
A number of different methods were employed. The most common was phlebotomy or venesection (often called "breathing a vein"), in which blood was drawn from one or more of the larger external veins, such as those in the forearm or neck. In arteriotomy an artery was punctured, although generally only in the temples. In scarification (not to be confused with scarification, a method of body modification) the "superficial" vessels were attacked, often using a syringe, a spring-loaded lancet, or a glass cup that contained heated air, producing a vacuum within.  A scarificator is a bloodletting tool used primarily in 19th century medicine. Leeches could also be used. The withdrawal of so much blood as to induce syncope (fainting) was considered beneficial, and many sessions would only end when the patient began to swoon.
William Harvey disproved the basis of the practice in 1628, and the introduction of scientific medicine, la methode numerique, allowed Pierre Charles Alexandre Louis to demonstrate that phlebotomy was entirely ineffective in the treatment of pneumonia and various fevers in the 1830s. Nevertheless, in 1840 a lecturer at the Royal College of Physicians would still state that "blood-letting is a remedy which, when judiciously employed, it is hardly possible to estimate too highly" and Louis was dogged by the sanguinary Broussais, who could recommend leeches fifty at a time.
Bloodletting was especially popular in the young United States of America, where Benjamin Rush (a signatory of the Declaration of Independence) saw the state of the arteries as the key to disease, recommending levels of blood-letting that were high, even for the time. George Washington was treated in this manner following a horseback riding accident: almost 4 pounds (1.7 litres) of blood was withdrawn, contributing to his death by throat infection in 1799.
One reason for the continued popularity of bloodletting (and purging) was that, while anatomical knowledge, surgical and diagnostic skills increased tremendously in Europe from the 17th century, the key to curing disease remained elusive and the underlying belief was that it was better to give any treatment than nothing at all. The psychological benefit of bloodletting to the patient (a placebo effect) may sometimes have outweighed the physiological problems it caused. Bloodletting slowly lost favour during the 19th century, but a number of other ineffective or harmful treatments were available as placebosmesmerism, various processes involving the new technology of electricity, many potions, tonics, and elixirs.
In the absence of other treatments bloodletting actually is beneficial in some circumstance, including the fluid overload of heart failure, and possibly simply to reduce blood pressure. In other cases, such as those involving agitation, the reduction in blood pressure might appear beneficial due to the sedative effect. In 1844 Joseph Pancoast listed the advantages of bloodletting in "A Treatise on Operative Surgery". Not all of these reasons are outrageous nowadays:

# Phlebotomy
Today it is well-established that bloodletting is not effective for most diseases, or at best less effective than modern treatments. Bloodletting still has its place in the treatment of a few diseases, including hemochromatosis and polycythemia; it is practiced by specifically trained practitioners in hospitals, using modern techniques.
In most cases, phlebotomy now refers to the removal of small quantities of blood for the purpose of performing blood tests. For more details on this subject, see Phlebotomy (modern).