Reviewing the research on teachers' diagnostic competences shows that most findings focus on the correspondence between teachers' diagnostic judgments and students' actual achievement, while cognitive processes and cognitive resources of teachers in diagnostic situations have been examined much less. We intend to extend this state of research from a domain-specific point of view by empirically identifying and theoretically describing processes and resources of mathematics teachers while judging tasks and students' solutions. In an interview study with expert teachers and mathematics educators (n=6) it was possible to deduce typical steps in a diagnostic process to identify resources (i.e. aspects of teacher knowledge) they relied on. [For the complete proceedings, see ED597799.]