Fasting is abstinence from food or drink, or both for health, ritualistic, religious, or ethical purposes. The abstention may be complete or partial, lengthy, of short duration, or intermittent. Fasting has been promoted and practiced from antiquity worldwide by physicians, by the founders and followers of many religions, by culturally designated individuals, and by individuals or groups as an expression of protest against what they believe are violations of social, ethical, or political principles.

Fasting has been used therapeutically since at least the 5th century BCE when Greek physician Hippocrates recommended abstinence from food or drink for patients who exhibited certain symptoms of illness. Some physicians recognized a fasting instinct, whereby patients in certain disease states naturally experience a loss of appetite. Some physicians believed that administering food during such states was unnecessary and possibly even detrimental since fasting was thought to be an important natural part of the recovery process.

An understanding of the physiological effects of fasting began to evolve in the latter part of the 19th century when some of the first organized studies of fasting were carried out in animals and humans. In the 20th century, as more became known about nutrition and the nutritional requirements of the human body, methods of fasting became increasingly sophisticated, and a wide array of approaches emerged. For example, fasting was used as a treatment and as a form of disease prevention, and it was carried out in various settings (e.g., in a hospital or clinic or at home).

Other methods, generally referred to as modified fasting, allowed for the intake of 200 to 500 kilocalories per day (daily calorie needs of adults range from about 1,600 to 3,000 kilocalories, depending on sex, age, and activity level) and sometimes included psychological or spiritual therapy; depending on the particular method used, calories usually were in the form of bread, vegetable broth, fruit juice, honey, or milk. Modified fasting was distinguished from a very low-calorie diet, which allowed up to 800 kilocalories per day and typically was aimed at inducing substantial weight loss.

The month of Ramadan in Islam is a period of penitence and total fasting from dawn to dusk. In addition to its role in religion, fasting may be used to express social and political views, particularly as a gesture of protest or solidarity.