The purpose of this explanatory case study was to elaborate on Desimone's professional development framework which holds that effective professional development should include five core features: content focus, active learning, coherence, duration, and collective participation. It did so by investigating the perceptions, professional learning, and instructional enactments of middle school science teachers as they participated in a yearlong professional development project designed to include the core features of Desimone's framework and which focused on the use of instructional strategies and scaffolds to facilitate student collaborative discourse, explanation, and argumentation in inquiry-based science investigations. The study also sought to investigate the ways participants adapted and enacted the instructional strategies put forth in the professional development in the classroom. The collective experiences of six middle school science teachers formed the case which was bounded by the beginning and end of the yearlong professional development project. This case study addressed the pressing problem of providing professional development to science teaches that is effective in shifting their instructional practice to better facilitate student discourse, explanation, and argumentation in inquiry-based science. The study responds to the suggestion that professional development targeting the use of strategies and scaffolds be used to support teachers in improving the effectiveness of small group discussions in inquiry-based science investigations. Data from interviews, professional development evaluations, classroom observations, teacher-created artifacts, lesson debriefs, and researcher memos indicate that participants perceived that collective participation and active learning were the most helpful core features of Desimone's framework. Content focus and duration were also reported to be helpful features but served mainly to support collective participation and active learning. Although participants perceived a low level of coherence between the goals of the professional development and their school's goals, this factor was not reported as helping or hindering in participants' use of strategies and scaffolds with students. Therefore, the role of coherence in Desimone's model remains unclear. While participants enacted and adapted discourse strategies in similar ways, they varied in how they implemented discourse scaffolds. Decisions to use discourse scaffolds were based on participants' perceptions of students' learning needs. When students were not required to use discourse scaffolds their conversations became less collaborative. In this study only the linkage of the core features of Desimone's model with increased teacher knowledge and skills; change in attitudes and beliefs; and change in classroom instruction were investigated. Future work should investigate how these elements affect student learning. The linkage of coherence with other core features and with support at the school level warrants further investigation. The use of objective tools to support teachers in making decisions about when students' need to use discourse scaffolds may also be a useful topic for further investigation. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]