<p>This study explored the effects of a yearlong writing intervention on students’ writing achievement, efficacy, and knowledge of cognitive strategies. The intervention was implemented by teachers in three districts who received professional development in using cognitive, sociocognitive, and disciplinary approaches to improve student literacy in history, specifically emphasizing source-based argument writing. We used a quasiexperimental research design to estimate the effects on student writing outcomes for students (n = 867) in grades 6–11 in the Southwest United States. Specifically, we compare outcomes of students whose teachers were in 1 of 3 conditions: randomly selected treatment (n = 12) or control teachers (n = 15) and self-selected teachers participating in their second year of the intervention (n = 15). Controlling for student demographics, prior achievement, and English language status, we find no significant difference between student writing quality for control and treatment teachers. However, the moderate effects of the intervention were significant (Cohen’s d = 0.29) for students whose teacher previously participated in a pilot year of the intervention and opted to participate for a second year, suggesting the value of additional practice in integrating the strategies into their curriculum and implementing the intervention. Across treatment and returning teachers, fidelity to the intervention significantly predicted student writing improvement. Results indicate the potential positive effects of a cognitive strategies approach to writing instruction. Implications for policy, practice, and future research are discussed.</p>