Homeopathy (also spelled homoeopathy or 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". 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.

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.

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”.