Writing is an essential skill for participating in modern American society. Although it is crucial to careers and civic engagement, student writing falls far short of national expectations (College Board, 2004; NCES, 2012; Persky, Daane, & Jin, 2003). The Common Core State Standards (CCSS) seek to increase the rigor of writing instruction students receive and also to increase the proportion of instruction focused on students' abilities to write argument and informational text because of their importance for success in college and careers (Calkins, Ehrenworth, & Lehman, 2012; Rothman, 2011; Cutler & Graham, 2008). With these new and loftier goals for student writing outcomes, practitioners and program developers are seeking guidance on how to help teachers work with students to obtain them. Teacher professional development is a logical approach. Well developed theory about instructional capacity (e.g., Cohen and Ball, 1999) suggests that student learning is produced by the interactions between teachers, students, and educational material. As a result, programs that combine professional development and aligned curriculum may be more likely to change teacher practice enough to improve student outcomes. In this context, the National Writing Project (NWP) won an Investing in Innovation (i3) grant to provide professional development to secondary teachers in rural districts with the goal of improving teachers' ability to teach to Common Core writing standards and ultimately students' writing proficiency. This paper presents a district-randomized controlled trial that found positive impacts of the NWP's College-Ready Writers Program (CRWP) using a validated measure of students' text-based argument writing as the outcome measure. This study adds rigorous experimental evidence to a body of literature that indicates that professional development that focuses on more than one aspect of instructional capacity--in this case, CRWP attended to both teachers and instructional materials--is more likely to lead to meaningful student learning. As such, the findings presented here have important implications for practitioners, policymakers, and program developers. Tables and figures are appended. [SREE documents are structured abstracts of SREE conference symposium, panel, and paper or poster submissions.]