This article focuses on the impacts of a program designed to prepare elementary classroom teachers to mentor preservice teachers for effective science instruction. The mentor preparation program was grounded in research on effective science instruction and learning-focused mentoring. The goals of the mentoring professional development were to (a) develop mentors' knowledge of what is important to talk about in effective mentoring conversations and (b) develop their skills with regard to how to structure and facilitate effective mentoring conversations about observed science lessons enacted by the mentees. We describe the impacts of the program on mentors' and mentees' beliefs about effective science instruction and their ability to facilitate and engage in effective mentoring conversations. Our analysis reveals that cooperating teachers who participated in the professional development sequence showed statistically significant increases in their beliefs about effective science instruction and their ability to facilitate effective mentoring conversations. Furthermore, the preservice mentees showed statistically greater gains in their beliefs about effective science instruction than their nonmentored peers.