Considering the relevance of teachers' diagnostic competence for understanding students' thinking and providing effective learning opportunities, Macarena Larrain investigates the development of future primary school teachers' diagnostic competence in error situations already during initial teacher education. Using video vignettes of classroom situations and samples of students' work, the author focuses on fostering future teachers' competence to identify students' errors, elaborate hypotheses about the causes of those errors and to design appropriate strategies for supporting students in overcoming their misconceptions. She also describes aspects of teachers' knowledge, beliefs and experience that are relevant for the competence and its development.