SRI’s 2-year random assignment evaluation found consistent program implementation and positive impacts of the National Writing Project’s College-Ready Writers Program (CRWP). Despite the challenge of implementing a program in 22 districts across 10 states delivered by 12 Writing Project sites, CRWP was implemented with a high degree of fidelity to key program components. Teachers in CRWP districts took up the materials and approaches presented by the program, and as a result the writing instruction that students experienced in treatment districts was significantly different from that in control districts. Ultimately, CRWP had a positive, statistically significant effect on the four attributes of student argument writing—content, structure, stance, and conventions—measured by the National Writing Project’s Analytic Writing Continuum for Source-Based Argument. In particular, CRWP students demonstrated greater proficiency in the quality of reasoning and use of evidence in their writing.