Homeopathy (also homœopathy or homoeopathy; from the Greek ὅμοιος, hómoios, "similar" + πάθος, páthos, "suffering" or "disease") is a controversial form of complementary and alternative medicine created in the late 18th century by German physician Samuel Hahnemann.The principles of homeopathy were laid out in his textbook, The Organon of the Healing Art, which remains in wide use today.

Homeopathic remedies are made from substances that, in undiluted form, cause symptoms similar to the disease they aim to treat.These substances are serially diluted, with shaking at each stage, that homeopaths believe removes side-effects from those that may be toxic, "adds to their power to stimulate a response", and "develops the special properties of the remedy" - even in those that are chemically inert or past the point where any molecules of the original substance remain.Hahnemann proposed that this process aroused and enhanced "spirit-like medicinal powers held within a drug". The therapeutic applications of the remedies used in homeopathy are recorded in homeopathic materia medica, and practitioners select treatments according to a patient consultation that explores both the physical and psychological state of the patient.

The ideas of homeopathy are scientifically implausible and directly opposed to modern pharmaceutical knowledge. Claims for the efficacy of homeopathy are unsupported by the collected weight of scientific and clinical studies. This lack of convincing evidence supporting its efficacy, along with its stance against modern scientific ideas, have caused homeopathy to be regarded as "placebo therapy at best and quackery at worst" in the words of a recent medical review. Various publications using meta-analysis, a common approach to pooling the results of many studies, reported positive results from the use of homeopathy. Facing difficulty in controlling for publication bias and the flawed designs of the studies they analyzed, these reports were regarded as inconclusive and unconvincing. A 2005 meta-analysis published in The Lancet comparing homeopathic clinical trials with those of conventional medicine demonstrated that homeopathy's effects are unlikely to be different from those of a placebo. Homeopaths are also accused of giving 'false hope' to patients who might otherwise seek effective conventional treatments. Many homeopaths advise against standard medical procedures such as vaccination, and some homeopaths even advise against the use of anti-malarial drugs.

The legal status of homeopathy varies from country to country, but homeopathic remedies are generally not tested and regulated under the same laws as conventional drugs. Usage is also variable and ranges from only two percent of people in Britain and the United States using homeopathy in any one year,to India, where homeopathy now forms part of traditional medicine and is used by approximately 15 percent of the population.