This paper describes a collaboration between a mathematician and a compositionist who developed a sequence of collaborative writing assignments for calculus. This sequence of developmentally appropriate assignments presents peer review as a collaborative process that promotes reflection, deepens understanding, and improves exposition. First, we distinguish writing-to-learn from writing-in-the-disciplines. Then, we review collaborative writing pedagogies and explain best practices for teaching peer review. Finally, we present an implementation plan and examples of student work that illustrate improved understanding of content and improved exposition.