In Western culture, alternative medicine is any healing practice "that does not fall within the realm of conventional medicine", or "that which has not been shown consistently to be effective." Alternative medicine is often based on the belief that a particular health regimen has efficacious effects even while there exists various bodies of evidence to contradict such a belief under the rigorous standards of evidence based medicine. In practice, alternative medicine encompasses therapies with a historical or cultural, rather than a scientific, basis. Commonly cited examples include naturopathy, chiropractic, herbalism, traditional Chinese medicine, Unani, Ayurveda, meditation, yoga, biofeedback, hypnosis, homeopathy, acupuncture, and diet-based therapies, in addition to a range of other practices. It is frequently grouped with complementary medicine, which generally refers to the same interventions when used in conjunction with mainstream techniques, under the umbrella term complementary and alternative medicine, or CAM. Some significant researchers in alternative medicine oppose this grouping, preferring to emphasize differences of approach, but nevertheless use the term CAM, which has become standard.

Alternative medicine practices are as diverse in their foundations as in their methodologies. Practices may incorporate or base themselves on traditional medicine, folk knowledge, spiritual beliefs, or newly conceived approaches to healing. Jurisdictions where alternative medical practices are sufficiently widespread may license and regulate them. The claims made by alternative medicine practitioners are generally not accepted by the medical community because evidence-based assessment of safety and efficacy is either not available or has not been performed for many of these practices. If scientific investigation establishes the safety and effectiveness of an alternative medical practice, whereupon it becomes mainstream medicine and is no longer "alternative", and will therefore become widely adopted by conventional practitioners. Because alternative techniques tend to lack evidence, some have advocated defining it as non-evidence based medicine, or not medicine at all. Some researchers state that the evidence-based approach to defining CAM is problematic because some CAM is tested, and research suggests that many mainstream medical techniques lack solid evidence.

A 1998 systematic review of studies assessing its prevalence in 13 countries concluded that about 31% of cancer patients use some form of complementary and alternative medicine. Alternative medicine varies from country to country. Dr. Edzard Ernst believes that in Austria and Germany CAM is mainly in the hands of physicians, while some estimates suggest that at least half of American alternative practitioners are physicians. In Germany, herbs are tightly regulated, with half prescribed by doctors and covered by health insurance based on their Commission E legislation.