Homeopathy (also spelled homoeopathy or homœopathy) is a form of alternative medicine, first proposed by German physician Samuel Hahnemann in 1796, in which practitioners use highly diluted preparations. Based on an ipse dixit axiom  formulated by Hahnemann, which he called the law of similars, preparations which cause certain symptoms in healthy individuals are given in diluted form to patients exhibiting similar symptoms. 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. Homeopaths call this process potentization. Dilution often continues until none of the original substance remains.

Apart from the symptoms, 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, with rare exceptions, considered safe  though homeopathy has been criticized for putting patients at risk due to advice against conventional medicine such as vaccinations, anti-malarial drugs, and antibiotics.

Homeopathy's efficacy beyond the placebo effect is unsupported by the collective weight of scientific and clinical evidence.  While some individual studies have positive results, systematic reviews of published trials fail to demonstrate efficacy conclusively.   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. A 2010 inquiry into the evidence base for homeopathy conducted by the United Kingdom's House of Commons Science and Technology Committee concluded that homeopathy is no more effective than placebo.

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 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".

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 coverage 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.