In each issue of "JSD," Joellen Killion explores a recent research study to help practitioners understand the impact of particular professional learning practices on student outcomes. In this issue, she describes Z. Kisa and R. Correnti's 2015 longitudinal study of fidelity of implementation of principles of reform-aligned professional development content and process. In the study Kisa and Correnti examine teacher reform-aligned teaching practices at the end of the fourth year of implementation of writing instruction improvements in 31 America's Choice schools. Using survey data of teacher perceptions collected from teachers more than a decade ago, the researchers measured changes in implementation fidelity of professional development content and process as well as frequency of teacher implementation of reform-aligned instructional practice. The study's findings showed overall, participating schools implemented the professional development content and process reforms at differing levels of fidelity, which influenced the degree of change in reform-aligned teaching practice. The study concludes that only high-high schools (those exhibiting high growth in both content and process) were successful in increasing the frequency of teachers' enactment of reform-aligned teaching practices over the course of the study's four years, while all other schools experienced consistent tapering off of reform-aligned teaching practices over the course of the study.