The National Institutes of Health (NIH) is an agency of the United States Department of Health and Human Services and is the primary agency of the United States government responsible for biomedical and health-related research.

The Institutes are responsible for 28%—about $28 billion—of the total biomedical research funding spent annually in the U.S., with most of the rest coming from industry. blank">Medical Research Spending Doubled Over Past Decade, Neil Osterweil, MedPage Today, September 20, _2005. The NIH is divided into two parts: the "Extramural" parts of NIH are responsible for the funding of biomedical research outside of NIH, while the "Intramural" parts of NIH conduct research. Intramural research is primarily conducted at the main campus in Bethesda in unincorporated Montgomery County, Maryland, and the surrounding communities. The National Institute of Aging and the National Institute on Drug Abuse are located in Baltimore, Maryland, and the National Institute of Environmental Health Sciences is in Research Triangle, North Carolina. The National Institute of Allergy and Infectious Diseases NIAID maintains Rocky Mountain Labs in Hamilton, Montana, blank">About NIAID, _NIAID web page, date accessed 14 January 2007. with an emphasis on virology.

The predecessor of the NIH began in 1887 as the Laboratory of Hygiene. It grew and was reorganized in 1930 by the Ransdell Act into the National Institute of Health (singular at the time). Today it is one of the world's foremost medical research centers, and the Federal focal point for medical research in the U.S. The NIH, comprising 27 separate institutes, centers and the Office of the Director, is part of the United States Department of Health and Human Services. The current NIH Director is Elias Zerhouni.

The goal of NIH research is to acquire new knowledge to help prevent, detect, diagnose, and treat disease and disability, from the rarest genetic disorder to the common cold. The NIH mission is to uncover new knowledge that will lead to better health for everyone. NIH works toward that mission by: conducting research in its own laboratories; supporting the research of non-Federal scientists in universities, medical schools, hospitals, and research institutions throughout the country and abroad; helping in the training of research investigators; and fostering communication of medical and health sciences information.