Homeopathy is a form of alternative medicine in which practitioners treat patients using highly diluted preparations that are believed to cause healthy people to exhibit symptoms that are similar to those exhibited by the patient. The collective weight of scientific evidence has found homeopathy to be no more effective than a placebo.

The basic principle of homeopathy, known as the law of similars, is let like be cured by like. It was first stated by German physician Samuel Hahnemann in 1796. Homeopathic remedies are prepared by serial dilution with shaking by forceful striking on an elastic body, which homeopaths term succussion. Each dilution followed by succussion is assumed to increase the effectiveness. Homeopaths call this process potentization. Dilution often continues until none of the original substance remains. Apart from the symptoms, homeopaths examine aspects of the patient's physical and psychological state, then homeopathic reference books known as repertories are consulted, and a remedy is selected based on the totality of symptoms.

While some individual studies have positive results, systematic reviews of published trials fail to demonstrate efficacy. Furthermore, higher quality trials tend to report results that are less positive, and most positive studies have not been replicated or show methodological problems that prevent them from being considered unambiguous evidence of homeopathy's efficacy.

Depending on the dilution, homeopathic remedies may not contain any pharmacologically active molecules, and for such remedies to have pharmacological effect would violate fundamental principles of science. Modern homeopaths have proposed that water has a memory that allows homeopathic preparations to work without any of the original substance; however, there are no verified observations nor scientifically plausible physical mechanisms for such a phenomenon. The lack of convincing scientific evidence to support homeopathy's efficacy and its use of “remedies” lacking active ingredients have caused homeopathy to be described as pseudoscience, quackery, and a cruel deception. Homeopathic remedies are safe at high dilutions recommended by Hahnemann, since they likely contain no molecules of the original substance, but they may not be safe at lower dilutions. Homeopathy has been criticized for putting patients at risk due to advice against conventional medicine such as vaccinations, anti-malarial drugs, and antibiotics.

The regulation and prevalence of homeopathy is highly variable from country to country. There are no specific legal regulations concerning its use in some countries, while in others, licenses or degrees in conventional medicine from accredited universities are required. In several countries, homeopathy is covered by the national insurance to different extents, while in some it is fully integrated into the national health care system. In many countries, the laws that govern the regulation and testing of conventional drugs do not apply to homeopathic remedies.

Homeopathy is a vitalist philosophy which interprets diseases and sickness as caused by disturbances in a hypothetical vital force or life force. It sees these disturbances as manifesting themselves as unique symptoms. Homeopathy maintains that the vital force has the ability to react and adapt to internal and external causes, which homeopaths refer to as the law of susceptibility. The law of susceptibility implies that a negative state of mind can attract hypothetical disease entities called miasms to invade the body and produce symptoms of diseases. However, Hahnemann rejected the notion of a disease as a separate thing or invading entity and insisted that it was always part of the living whole. Hahnemann proposed homeopathy in reaction to the state of traditional western medicine at that time, which often was brutal and more harmful than helpful. Hahnemann coined the expression "allopathic medicine" to pejoratively refer to traditional western medicine.