The adoption of the Common Core State Standards in English Language Arts (CCSS-ELA) has brought a new prominence to writing instruction. Additionally, the CCSS-ELA increase the focus on argument and informational/explanatory writing. To facilitate elementary teachers’ transition to this new emphasis on writing in general, and argument and informational writing specifically, the National Writing Project was awarded a 1-year U.S. Department of Education Supporting Effective Educator Development (SEED) grant. This grant enabled the National Writing Project to provide intensive, sustained in-service professional development to high-need elementary schools focused on CCSS-ELA implementation in third, fourth, and fifth grades. SRI conducted a cluster randomized controlled trial to estimate the effects of the SEED professional development on teachers’ writing instruction and student argument writing, while documenting the implementation of the SEED program and the contexts in which the program was executed. This report describes SRI’s findings.