The purpose of this study is to identify mathematicians’ beliefs and practices in evaluating students’ written mathematical proofs and teaching students to write proofs. Thus, this study addresses the call for research on mathematicians' pedagogical practices in advanced mathematics courses. Mathematicians were interviewed and asked to (a) evaluate proofs written by students, (b) discuss the characteristics they view as constituting a well-written proof, and (c) discuss their teaching practices with respect to proof.