In its Iran-Contra investigation, Congress faced legal challenges which evolved logically from the two-century long history of Congressional investigations, and yet at the same time were unmatched in their significance. One challenge concerned "use" immunity, and its employment when high advisers to the President faced parallel Congressional and criminal proceedings. A second concerned the investigation of a secret international "Enterprise," which, like similar enterprises, was established overseas to carry on international operations without public accountability, and was protected by multiple layers of secrecy sanctioned by law. This article addresses how history brought Congress to those challenges, and how Congress met them, because they are the two major legal obstacles to contemporary Congressional investigations. Part One begins with a discussion of factors influencing the timing of congressional investigations and the conflict between such investigations and parallel or subsequent criminal investigations and prosecutions. It then analyzes key developments in the law of immunity since Watergate. Next, this part reviews the employment of use immunity in the Watergate and Iran-Contra congressional investigations. After a general treatment of the employment of use immunity in those investigations, the article considers two types of problems with the use of the federal immunity statute: first, significant interpretive problems under the statute and second, problems in enforcing immunity orders. Part Two begins with a survey of the expanding use of international enterprises whose operations are deliberately concealed from United States executive and congressional authorities by a web of foreign secrecy laws. As an example of such concealment, it then describes the legal artifices used to create and hide the North-Secord Enterprise and the operation of that Enterprise in the Iran-Contra Affair. Next, Part Two considers the strengths and weaknesses of the three basic legal tools available to the United States in investigating such enterprises: treaties, compelled confidentiality waivers, and compelled or negotiated transfer of information through immunization or other means. It analyzes the use of these tools in the Iran-Contra investigation. Part Two then considers the overseas reach of United States process for individual and corporate evidence, the barriers created by bank secrecy laws, and techniques of international investigation.