Proof is a fundamental aspect of mathematics. However, in the high school curriculum, it often receives uneven attention that is focused on form rather than understanding. One avenue for addressing this issue is to change and strengthen teachers' conceptions of proof. To explore this idea, we followed a group of teachers as they participated in a summer mathematics research experience. During this experience, proof was not an isolated exercise but part of the mathematical process of discovery. In this study, we analyzed pre- and post-survey data and participants' critique of proofs to uncover the influence of the mathematics research experience on their concept of proof. We present data on the criteria participants used to evaluate proofs, their conception of proof, and how the mathematics research experience changed their conception of proof. [For the complete proceedings, see ED657822.]