In this randomized controlled study, 20 second- and third-grade teachers participated in practice-based professional development in strategies instruction in either story or opinion essay writing, using the Self-Regulated Strategies Development (SRSD) instructional model. These teachers worked in schools collaborating with a local university to implement an evidence-based, three-tiered model of prevention and supports targeting academic, behavioral, and social goals. The authors examined the effects of intensive practice-based professional development and follow-up support on the writing of second- and third-grade students in terms of quality, length, and inclusion of basic genre elements; integrity of SRSD instruction; and teacher and student judgments of the social validity of SRSD. Whole-class (Tier 1), teacher-implemented SRSD instruction resulted in significant and meaningful changes in student writing outcomes for story and opinion essay writing. Teachers implemented SRSD with fidelity, and SRSD was viewed as socially valid by teachers and students. Limitations of this study and directions for future research are discussed. (Contains 5 tables.)