Chiropractic is a health care discipline and profession that emphasizes diagnosis, treatment and prevention of mechanical disorders of the musculoskeletal system, especially the spine, under the hypothesis that these disorders affect general health via the nervous system.

It is generally considered to be complementary and alternative medicine, a characterization that many chiropractors reject. The main treatment involves manual therapy including manipulation of the spine, other joints, and soft tissue; treatment also includes exercises and health and lifestyle counseling. Traditional chiropractic assumes that a vertebral subluxation or spinal joint dysfunction interferes with the body's function and its innate intelligence, a notion that brings ridicule from mainstream science and medicine.

D.D. Palmer founded chiropractic in the 1890s and his son B.J. Palmer helped to expand it in the early 20th century. It has two main groups: "straights", now the minority, emphasize vitalism, innate intelligence and spinal adjustments, and consider subluxations to be the leading cause of all disease; "mixers" are more open to mainstream and alternative medical techniques such as exercise, massage, nutritional supplements, and acupuncture. Chiropractic is well established in the U.S., Canada and Australia.

For most of its existence, chiropractic has battled with mainstream medicine, sustained by ideas such as subluxation that are not based on solid science. Vaccination remains controversial among chiropractors. The American Medical Association called chiropractic an "unscientific cult" and boycotted it until losing a 1987 antitrust case. Chiropractic has had a strong political base and sustained demand for services; in recent decades, it has gained more legitimacy and greater acceptance among medical physicians and health plans in the U.S., and evidence-based medicine has been used to review research studies and generate practice guidelines. Many studies of treatments used by chiropractors have been conducted, with conflicting results. Collectively, systematic reviews of this research have not demonstrated that spinal manipulation is effective, with the possible exception of treatment of back pain. The efficacy and cost-effectiveness of maintenance chiropractic care are unknown. Although spinal manipulation can have serious complications in rare cases, chiropractic care is generally safe when employed skillfully and appropriately.

Chiropractic was founded in the 1890s by Daniel David (D.D.) Palmer in Davenport, Iowa. Palmer, a magnetic healer, hypothesized that manual manipulation of the spine could cure disease. Chiropractic competed with its predecessor osteopathy, another medical system based on magnetic healing and bonesetting; both systems were founded by charismatic midwesterners in opposition to the conventional medicine of the day, and both postulated that manipulation improved health. Although initially keeping chiropractic a family secret, in 1898 Palmer began teaching it to a few students at his new Palmer School of Chiropractic. One student, his son Bartlett Joshua (B.J.) Palmer, became committed to promoting chiropractic, took over the Palmer School in 1906, and rapidly expanded its enrollment.

Early chiropractors believed that all disease was caused by interruptions in the flow of innate intelligence, a vital nervous energy or life force that represented God's presence in man; chiropractic leaders often invoked religious imagery and moral traditions. D.D. and B.J. both seriously considered declaring chiropractic a religion, which might have provided legal protection under the U.S. constitution, but decided against it partly to avoid confusion with Christian Science. Early chiropractors also tapped into the Populist movement, emphasizing craft, hard work, competition, and advertisement, aligning themselves with the common man against intellectuals and trusts, among which they included the American Medical Association (AMA).

B. J. Palmer stated that:

Chiropractic has seen considerable controversy and criticism. Although D.D. and B.J. were "straight" and disdained the use of instruments, some early chiropractors, whom B.J. scornfully called "mixers", advocated use of instruments. In 1910 B.J. changed course and endorsed X-rays as necessary for diagnosis; this resulted in a significant exodus from the Palmer School of the more conservative faculty and students. The mixer camp grew until by 1924 B.J. estimated that only 3,000 of the U.S.'s 25,000 chiropractors remained straight. That year, B.J.'s promotion of the neurocalometer, a new temperature-sensing device, was another sign of chiropractic's gradual acceptance of medical technology, although it was highly controversial among B.J.'s fellow straights. By the 1930s chiropractic was the largest alternative healing profession in the U.S.

Chiropractors faced heavy opposition from organized medicine. Thousands of chiropractors were prosecuted for practicing medicine without a license, and D.D. and many other chiropractors were jailed. To defend against medical statutes B.J. argued that chiropractic was separate and distinct from medicine, asserting that chiropractors "analyzed" rather than "diagnosed", and "adjusted" subluxations rather than "treated" disease. B.J. cofounded the Universal Chiropractors' Association (UCA) to provide legal services to arrested chiropractors. Although the UCA won their first test case in Wisconsin in 1907, prosecutions instigated by state medical boards became increasingly common and in many cases were successful. In response, chiropractors conducted political campaigns to secure separate licensing statutes, eventually succeeding in all fifty states, from Kansas in 1913 through Louisiana in 1974. The longstanding feud between chiropractors and medical doctors continued for decades. The AMA labeled chiropractic an "unscientific cult" in 1966, and until 1980 held that it was unethical for medical doctors to associate with "unscientific practitioners". This culminated in a landmark 1987 decision, Wilk v. AMA, in which the court found that the AMA had engaged in unreasonable restraint of trade and conspiracy, and which ended the AMA's de facto boycott of chiropractic. In 2008 and 2009, chiropractors used libel lawsuits and threats of lawsuits against their critics.

Serious research to test chiropractic theories did not begin until the 1970s, and is continuing to be hampered by what are characterized as antiscientific and pseudoscientific ideas that sustained the profession in its long battle with organized medicine. By the mid 1990s there was a growing scholarly interest in chiropractic, which helped efforts to improve service quality and establish clinical guidelines that recommended manual therapies for acute low back pain.[26] In recent decades chiropractic gained legitimacy and greater acceptance by medical physicians and health plans, and enjoyed a strong political base and sustained demand for services. However, its future seemed uncertain: as the number of practitioners grew, evidence-based medicine insisted on treatments with demonstrated value, managed care restricted payment, and competition grew from massage therapists and other health professions. The profession responded by marketing natural products and devices more aggressively, and by reaching deeper into alternative medicine and primary care.