The purpose of this study was to evaluate a two-year professional development (PD) initiative implemented in an elementary school with low literacy achievement in the primary grades. The PD system that was implemented combines the research-based components of effective literacy instruction with the necessary features for implementing change in schools. Nine teachers from Junior Kindergarten to Grade 3 participated in the intervention. The impact of the PD was assessed on students' letter/word recognition, letter/word spelling (conventional and invented), reading comprehension and written expression. The results revealed improvement in certain measures of students' literacy skills, particularly in spelling. However, the change was not uniform across grade levels and literacy measures. This variability highlights the important role that process and context variables play in mediating the impact of in-service PD on student outcomes.