This study examined the effects of professional development in the form of a teacher study group and a particular discussion approach, Collaborative Reasoning (CR) (Anderson, et al., 2001) upon teachers' questioning and students' levels of thinking during discussion of text. The study explored how five middle school teachers achieved a deeper understanding of teachers' authentic questioning and the facilitation of CR discussion. The Formative and Design Experiment (FandDE) framework was used in this investigation. Both qualitative and quantitative data were collected. Teachers coded transcripts of one baseline and four CR discussions for each teacher. Transcripts were coded for teachers' CR instructional moves, CR features incorporated by the students in discussion, and the levels of thinking supporting students' responses. Data indicated that the nature of discussions progressed from recitation to more dialogical discussion patterns. Teachers implemented more authentic questioning, and students were observed to use more higher-order thinking in the responses. Students discourse showed a higher incidence of exploratory talk and uptake. Students used multiple kinds of evidence from personal experience, texts, and knowledge from previous reading/lecture/discussion to support their arguments. The data suggest that the implementation of a specific discussion model may enhance teachers' questioning and encourage students to engage in higher-order thinking and reasoning when discussing text.