How are politicians informed and who do politicians seek information from? The role of information has been at the center for research on legislative organizations but there is a lack of systematic empirical work on the information that Congress seeks to acquire and consider. To examine the information flow between Congress and external groups, we construct the most comprehensive dataset to date on 74,082 congressional committee hearings and 755,540 witnesses spanning 1960-2018. We show descriptive patterns of how witness composition varies across time and committee, and how different types of witnesses provide varying levels of analytical information. We develop theoretical expectations for why committees may invite different types of witnesses based on committee intent, inter-branch relations, and congressional capacity. Our empirical evidence shows how certain institutional conditions can affect how much committees turn to outsiders for information and from whom they seek information.