Chapter 3 introduces an innovative measure of collaboration in Congress based on internal communications in Congress known as Dear Colleague letters (DCLs). The chapter begins with a brief history of DCLs, tracing their usage back to the early twentieth century and showing how members use them to advertise and build support for their ideas among their colleagues. These letters shed new light on the day-to-day work of Congress, as members send them in the earliest stages of the policy process and across a wide range of policymaking activities, from bill introductions to letters to the executive branch. When members sign a letter together, they are claiming joint ownership of its content, reflecting an underlying collaborative relationships. These signatures are used to create a network representation of the collaborative Congress that accounts for the inherently relational dynamics of collaboration in the House of Representatives.