Background/Context: Current efforts to build rigorous teacher evaluation systems has increased interest in standardized classroom observation tools as reliable measures for assessing teaching. However, many argue these instruments can also be used to effect change in classroom practice. This study investigates a model of professional development (PD) built around a tool--the Protocol for Language Arts Teaching Observations (PLATO). Purpose/Objective: The study analyzes the extent to which teachers appropriated the instructional practices targeted in the PLATO PD. We also assess factors that may have supported and/or hindered teachers' uptake of practices. Setting/Participants: The study sample includes 27 teachers who participated in PD over 2 years. Teachers worked in six middle schools in a single, large urban district. Intervention: The two year PD consisted of 5 daylong sessions each school year, and a 4-day summer institute. All sessions focused on the PLATO scales. Teachers also worked in school-based teams to design lessons featuring the focal practices and attended five school-site meetings with PLATO PD providers. Research Design and Data Collection: PLATO served as a set of practices around which to orient PD, as well as a standardized tool for measuring changes in teacher practice. All teachers were observed using PLATO scales throughout the PD and during the subsequent year. We conducted multiple interviews with all participating teachers, which were transcribed and coded by multiple researchers. Case studies of six purposively sampled teachers incorporate interviews, scores, and field notes. Findings/Results: The duration of PD mattered in terms of teachers' appropriation of PLATO practices. In addition, "foundational practices" supported the appropriation of more ambitious practices targeted in the PLATO PD, including time and behavior management and instructional planning. Finally, our findings suggest stable and collaborative communities support professional learning and growth. Conclusions/Recommendations: The findings suggest moving away from "one size fits all models" and differentiating PD for teachers. Effective professional development may not be effective for all teachers. Observation protocols can play a unique role in PD by allowing professional developers to gather standardized information across teachers and to compare changes in teacher practice in systematic ways. PD providers might also use such tools diagnostically to identify and respond to the heterogeneity in teachers' practice.