The project reported in this chapter aimed at assessing and fostering future teachers’ diagnostic competence in teaching and learning situations in which students’ errors occurred. Based on a model of diagnostic processes in error situations a university course and a pre- and post-test were developed. Probabilistic models (such as Item-Response-Theory and Latent-Class-Analysis) were used to assess the future teachers’ diagnostic competence in erroneous teaching and learning situations based on their answers to the test items. The results show that the university course had an influence on the way future teachers hypothesize about the causes of the students’ errors and partly also on their preferred way of dealing with errors. Additionally the importance of practical experience when fostering diagnostic competence in error situations became apparent.