The formation of accurate judgements on students’ performance is often considered as part of teachers’ professional competence. Moreover, inaccurate judgements are seen as determinants of social inequality. Using data of BiKS-8-18 on teacher ratings and student performance, the paper gives an overview on different theoretical approaches and operationalization of judgement accuracy as well as their results in terms of homogeneity, stability over time, inter-individual differences and the effects of judgement accuracy on students’ further achievement. Primary school teachers outperform secondary school teachers in accurately assessing student performance. Furthermore, judgement accuracy did not show to be a general ability. Applying to different student characteristics, however, related to subject areas/domains it proofed to be a relatively time persistent teacher ability. Teacher judgements are somewhat sensitive to characteristics at the class and student level, although bias related to students’ gender and social status was not found for teachers at secondary level. We found positive effects of teacher judgements on students’ achievement gains, particularly in the domain of reading. Finally, by taking into account an add-on study of teachers’ content related knowledge related to judgements on reading performance, we discuss the findings and further highlight the need to take into account judgement purposes and demands in future research.