This study examines how high-quality professional development can promote the diffusion of effective teaching strategies among teachers through collaboration. Drawing on longitudinal and sociometric data from a study of writing professional development in 39 schools, this study shows that teachers' participation in professional development is associated with providing more help to colleagues on instructional matters. Further, the influence of professional development on participants' instructional practice diffuses through the network of helping. These findings suggest that in addition to direct effects, spillover effects of professional development can occur through collegial interactions. Evidence presented in this study potentially helps educational leaders develop high-quality professional development programs and distribute professional development participants within schools to enhance all teachers' instructional practices. (Contains 14 notes, 7 tables, and 1 figure.)