Teacher judgments matter for students: They have an impact not only on students’ academic self-concepts and for their theories about themselves as learners but also on educational pathways through grades and school leaving certificates. However, do teachers have the expertise for such judgments? Is there such a thing as a professional ability to judge (diagnostic competence)? The essay provides an overview on research related to teacher judgment accuracy and bias and specifies conditions under which accurate teacher judgments can (and cannot) be expected. It is argued that in research as well as in teacher training, causes of judgment and judgment demands need to be taken into account and that the role of knowledge components in controlled and automatic decision-making after intensive experience on the job needs to be better understood.