National Institutes of Health, Federal health research agency in Bethesda, United States.

The National Institutes of Health is a federal health research agency in Bethesda made up of multiple specialized research centers. The campus includes laboratory buildings, research facilities, and a clinical center spread across a large site in Montgomery County.

The institution grew from a small laboratory and developed over more than a century into the largest public funder of biomedical research worldwide. The expansion of the campus occurred mainly after World War II, when interest in medical research increased sharply.

The plural name adopted in 1948 reflects the 27 institutes and centers counted in 2012, among them the National Cancer Institute, created in 1937, and the National Library of Medicine. The campus moved to Bethesda in 1938 from the Old Naval Observatory in Washington.

The NIH campus in Bethesda is a working federal research site, not a public attraction; there are no guided tours or exhibition spaces open to visitors. Access to the grounds and buildings is controlled, and research areas require authorization. The complex sits among curving roads in Montgomery County, northwest of Washington, D.C.

The Clinical Center is a research hospital where patients from around the world receive treatment while experimental therapies are tested. Over the decades, thousands of participants in clinical trials contributed to the development of new treatment methods.