Homeopathy (also spelled Homoeopathy or Homœopathy) is a form of alternative medicine, first proposed by German physician Samuel Hahnemann in 1796, that attempts to treat patients with heavily diluted preparations which are thought to cause effects similar to the symptoms presented. Homeopathic remedies are prepared by serial dilution with shaking by forceful striking, which homeopaths term "succussion," after each dilution under the assumption that this increases the effect of the treatment. Homeopaths call this process "potentization". Dilution often continues until none of the original substance remains.

Apart from the symptoms of the disease, homeopaths use aspects of the patient's physical and psychological state in recommending remedies. Homeopathic reference books known as repertories are then consulted, and a remedy is selected based on the totality of symptoms. Homeopathic remedies are considered safe, with rare exceptions. Some homeopaths have, however, been criticized for putting patients at risk with advice to avoid conventional medicine such as vaccinations, anti-malarial drugs, and antibiotics.

The prevalence and regulation 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. Some homeopathic treatment is covered by the national insurance coverage of several countries; in others 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.

Claims of homeopathy's efficacy beyond the placebo effect are unsupported by the collective weight of scientific and clinical evidence. While some studies have positive results, systematic reviews of all the published trials fail to conclusively demonstrate efficacy. Furthermore, higher quality trials tend to report less positive results, and most positive studies have not been replicated or show methodological problems that prevent them from being considered unambiguous evidence of homeopathy's efficacy.

Homeopathic remedies generally contain few or no 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, the physics of water are well understood, and no known mechanism permits such a memory. The lack of convincing scientific evidence supporting homeopathy's efficacy and its use of remedies lacking active ingredients have caused homeopathy to be described as pseudoscience, quackery, and a "cruel deception".