Academic studies have often emphasized the law-making aspects of Congress to the exclusion of examining how Congress uses its investigative power. This is despite the fact that Congress possesses great power to compel testimony and documents from public and private persons alike, and that exercises of the investigative power are among the most notable public images of Congress. While several recent studies have considered investigations in the context of relations between the executive and legislative branches, far less effort has been committed to looking at how much Congress uses coercive investigative power to gather information on non-governmental actors.I develop several new datasets to examine the historical and recent use of investigations of both governmental and non-governmental institutions. A major component of this work is a comprehensive study of all authorizations of subpoena power granted to committees over the period 1792-1944. I find that for both Executive and outside subjects, divided control of government was positively associated with the volume of investigations in the House, but not in the Senate. Through extended qualitative examination of contemporary news stories and other secondary sources, I consider how partisan and institutional factors influenced the focus, scope, and intensity of investigative projects, as well as which legislative chamber and committee came to undertake the information-gathering work. I demonstrate how stronger party leaderships limited opportunities for members to conduct investigations, and where necessary, shifted politically-sensitive subjects to venues--within the chamber, to the other chamber, or to joint committees--that would be more favorable to the desired results. Nevertheless, it was in the Senate that individual members consistently possessed greater opportunity to initiate inquiries to develop policy expertise as well as those that could be dangerous to their own party's interests. Confronting changes to how Congress has made such grants since World War II, I examine the exercise of investigative power in its capacity to generate media coverage and witnesses' invocations of the Fifth Amendment protection against self-incrimination. Whereas the House and Senate experienced an extended period of near-parity in the quantity of investigations until the mid-1990s, since that time the Senate has been the more involved and consistent actor. The number of different investigative topics receiving media attention has also declined since the early 1990s. Increasingly, Congress's investigative initiative focuses on a small set of subjects, which often receive simultaneous scrutiny from a number of different corners. To help motivate the evolution of Congressional investigations, I consider the Legislative branch's relations with the Executive and Judicial branches, as well as concerted attempts by the Legislature to build institutional capacity. I argue that the growth of the State and increased complexity of society have complicated Congress's investigative task, requiring it to seek new methods and driving it towards greater reliance on the other two branches, particularly the Judiciary. Ultimately, Congress's ability to summarily detain contumacious witnesses and police member qualifications and chamber legitimacy--the original means and ends of its powers as upheld in the 19th Century--are the weakest and most disused elements of its investigative repertoire. The work closes with a discussion of the possibilities for policy-making through investigations, and some ideas for encouraging greater member use of Congress's investigative power.