What is the impact of implementing standards-based writing instruction on teacher beliefs and practices? Although initially skeptical, middle school teachers who collaborated to develop and use common assessments, rubrics, and exemplars came to believe that these practices helped nearly all of their students achieve at higher levels. A key element of the change process was a high level of informal collaboration as a tool for professional development. Teachers worked together to develop common assessments, look at student work, select and share writing exemplars, and discuss instructional strategies in both informal and formal sessions. Implications for practice include the need for specific and structured support systems for the implementation of standards-based instruction and the need to design job-embedded professional development systems for adult learners. Additionally, school cultures that encourage collaboration are essential to successful implementation of standards-based teaching and learning.