The authority of Congress to investigate is an implied constitutional power. As James Madison explained in Federalist 51, "In framing a government which is to be administered by men over men…, you must first enable the government to control the governed; and in the next place oblige it to control itself." Congress has exercised this investigative power since the earliest days of the republic.
Beginning in 1792 Congress has conducted hundreds of investigations in order to inform the public and to write good legislation. Successful inquiries have required persistence, thoroughness, and an expert staff. The Senate has probed issues such as interstate commerce, Ku Klux Klan activities, the sinking of R.M.S. Titanic, Wall Street banking practices, organized crime, antiunion activity, the sale of cotton, and the Vietnam War. Perhaps the Senate's best-known investigatory committee, the Select Committee on Presidential Campaign Activities (commonly known as the Watergate Committee), investigated alleged malfeasance in the executive branch and was instrumental in bringing about the resignation of President Richard Nixon.
Some Notable Investigations: