In recent years, different studies have emphasized the necessity to improve prospective teachers’ professional knowledge and teaching competences, especially prospective teachers’ diagnostic competence, which accounts for perception, interpretation, and decision-making concerning students’ individual thinking processes and learning obstacles. In order to do so, this paper follows a content-related approach that focuses specifically on diagnostic judgments on conceptual understanding and procedural skills of students’ understanding of conditional probabilities and their underlying prior knowledge elements. Prospective teachers are known to face challenges in adequately focusing on these aspects in their diagnostic judgments. Therefore, written diagnostic judgments of 26 prospective teachers on two transcript vignettes are investigated. The results indicate that prospective teachers show a high focus on prior knowledge and on procedures in their diagnostic judgments.