For reasoning and proof to become a reality in mathematics classrooms, it is important to prepare teachers who have knowledge and skills to integrate reasoning and proving in their teaching. Aiming to enhance prospective secondary teachers’ (PSTs) content and pedagogical knowledge related to proof, we designed and studied a capstone course Mathematical Reasoning and Proving for Secondary Teachers. This paper describes the structure of the course, and illustrates how PSTs’ interacted with its different components by focusing on one of the four course modules: conditional statements (CS). The PSTs first strengthened their content knowledge of CS, then developed and taught in local schools a lesson incorporating CS. The data analysis show gains in PSTs’ knowledge for teaching proof and dispositions towards proving, following their participation in the course.