Writing development involves changes that occur in children's strategic behavior, knowledge, and motivation. The authors examined the effectiveness of self-regulated strategy development (SRSD), a strategy instructional model designed to promote development in each of these areas. Instruction focused on planning and writing stories and persuasive essays. The addition of a peer support component to SRSD instruction aimed at facilitating maintenance and generalization effects was also examined. SRSD had a positive impact on the writing performance and knowledge of struggling second-grade writers attending urban schools serving a high percentage of low-income families. In comparison with children in the Writers' Workshop condition, SRSD instructed students were more knowledgeable about writing and evidenced stronger performance in the two instructed genres (story and persuasive writing) as well as two uninstructed genres (personal narrative and informative writing). Moreover, the peer support component augmented SRSD instruction by enhancing specific aspects of students' performance in both the instructed and uninstructed genres. (Contains 7 tables.)