Congress, in addition to its lawmaking duties, has oversight authority over the Executive Branch.
Simply put, Congressional oversight is when members of Congress monitor Executive Branch activities.
The dictionary definition of "oversight" is "watchful care; superintendence; general supervision." Congress regards oversight as "the authority to conduct inquiries or investigations of the executive, to have access to records or materials held by the executive, or to issue subpoenas for documents or testimony from the executive."
Oversight is an implied rather than an enumerated power under the U.S. Constitution. The government's charter does not explicitly grant Congress the authority to conduct inquiries or investigations of the executive, to have access to records or materials held by the executive, or to issue subpoenas for documents or testimony from the executive.
There was little discussion of the power to oversee, review, or investigate executive activity at the Constitutional Convention of 1787 or later in the Federalist Papers, which argued in favor of ratification of the Constitution. The lack of debate was because oversight and its attendant authority were seen as an inherent power of representative assemblies which enacted public law.
Oversight also derives from the many and varied express powers of the Congress in the Constitution. It is implied in the legislature's authority, among other powers and duties, to appropriate funds, enact laws, raise and support armies, provide for a Navy, declare war, and impeach and remove from office the President, Vice President, and other civil officers. Congress could not reasonably or responsibly exercise these powers without knowing what the executive was doing; how programs were being administered, by whom, and at what cost; and whether officials were obeying the law and complying with legislative intent.
The Supreme Court made legitimate the oversight powers of Congress, subject to constitutional safeguards for civil liberties, on several occasions. In 1927, for instance, the High Court found that in investigating the administration of the Justice Department, Congress was considering a subject "on which legislation could be had or would be materially aided by the information which the investigation was calculated to elicit."