The Congressional Research Service (CRS) is a public policy research institute of the United States Congress. Operating within the Library of Congress, it works primarily and directly for members of Congress and their committees and staff on a confidential, nonpartisan basis. CRS is sometimes known as Congress' think tank due to its broad mandate of providing research and analysis on all matters relevant to national policymaking.

CRS has roughly 600 employees reflecting a wide variety of expertise and disciplines, including lawyers, economists, historians, political scientists, reference librarians, and scientists. In the 2016 fiscal year, it was appropriated a budget of roughly $106.9 million by Congress.

CRS was founded during the height of the Progressive Era as part of a broader effort to professionalize the government by providing independent research and information to public officials. Its work was initially made available to the public, but between 1952 and 2018 was restricted only to members of Congress and their staff; non-confidential reports have since been accessible on its website. In 2019, CRS announced it was adding "the back catalog of older CRS reports" and also introducing new publicly available reports, such as its "two-page executive level briefing documents."

CRS is one of three major legislative agencies that support Congress, along with the Congressional Budget Office (which provides Congress with budget-related information, reports on fiscal, budgetary, and programmatic issues, and analyses of budget policy options, costs, and effects) and the Government Accountability Office (which assists Congress in reviewing and monitoring the activities of government by conducting independent audits, investigations, and evaluations of federal programs). Collectively, the three agencies employ more than 4,000 people.