Given that the importance and place of proof in mathematics education has been one of the emphases in the reform movement, it is crucial for teachers to be well equipped to teach mathematical reasoning and proof. The purpose of this study was to examine how pre-service middle school teachers conceptions of proof evolved during a geometry content course for teachers. Six preservice teachers participated in this study. Multiple-case study method was used with each participant being the unit of analysis. Semi-structured interviews were conducted with the participants and the instructor of the course. Participants in this study recognized explaining why as an important role of proof and they developed an appreciation for the importance of proofs for themselves as teachers. The results also revealed that pre-service middle school teachers were unable to talk about the generality of proofs in the absence of a general argument if they had not developed a robust understanding of the fact that proofs prove for all cases. This study suggests that a nontraditional content class might help prospective teachers start valuing proofs and become aware of the explanatory power of proofs at least for themselves as teachers.