Legislative oversight is most commonly conducted through congressional budget, authorisation, appropriations, confirmation, and investigative processes, and, in rare instances, through impeachment. But the adversarial, often confrontational, and sometimes high profile nature of congressional investigations sets it apart from the more routine, accommodative facets of the oversight process experienced in authorisation, appropriations, or confirmation exercises. While all aspects of legislative oversight share the common goals of informing Congress so as to best accomplish its tasks of developing legislation, monitoring the implementation of public policy, and disclosing to the public how its government is performing, the inquisitorial process also sustains and vindicates Congress's role in our constitutional scheme of separated powers and checks and balances. The rich history of congressional investigations from the failed St Clair expedition in 1792 through Teapot Dome, Watergate, Iran-Contra, Whitewater, and the current ongoing inquiries into Operation Fast and Furious, has established, in law and practice, the nature and contours of congressional prerogatives necessary to maintain the integrity of the legislative role in that constitutional scheme. This book reviews the legal basis for investigative oversight, followed by several prominent examples of congressional oversight that reflects the significant breadth and reach of the legislative investigative prerogative vis-á-vis the Department.