The purpose of the study was to examine teacher talk during shared planning time to provide insight into the rationales behind teachers’ decision making that may be related to their underlying beliefs about subject matter, teaching, learning, and their students. A team of 4 eighth-grade mathematics teachers at a suburban middle school in the southern part of the United States agreed to participate in this study. The study supported the hypothesis that teachers’ collaborative planning time discourse provides a unique lens for understanding teachers’ beliefs.