Chapter 4 examines how collaboration varies across issue area and policy substance. Dear Colleague letters are classified into one of twenty issue areas based on the comparative agendas project coding scheme, and for each issue area, the proportions of letters that are noncollaborative, bipartisan, and partisan are identified. Examining why some issues are more collaborative than others reveals that collaboration – particularly bipartisan collaboration – is more common on issues that do not fall neatly onto a liberal–conservative scale, where compromise and common ground are easier to find, and on issues that are on the majority party agenda, where there are more opportunities to create policy by incorporating an idea into a larger, moving bill. The second part of the chapter considers the significance of collaborative policy and establishes that members of Congress routinely coauthor substantive policy proposals, and this is not a phenomenon limited to naming of post offices. These findings support the social exchange theory of collaboration by providing evidence of how expected costs and benefits shape the likelihood of collaboration.