There has been growing recognition of the high frequency with which Americans utilize complementary and alternative health care providers and that annually about 10% of the population is treated by a chiropractor. While chiropractors employ several types of treatment, spinal manipulation is the most important and unique of these therapies. Over 90% of the spinal manipulations performed in the U.S. are done by chiropractors with most of the remainder performed by osteopathic physicians. The annual number of chiropractic visits in the U.S. has been estimated at approximately 100 per 100 person-years.