Teacher effectiveness, which impacts student attainment even when controlling for student characteristics, is of key importance as a factor in educational effectiveness and improvement. Improving the quality of teaching is thus the primary means by which we can enhance student learning outcomes. Thus there has long been great interest in the development of classroom observation measures in the field of educational effectiveness research (EER). The International System for Teacher Observation and Feedback (ISTOF) is a unique instrument in the field, as it was developed by a team from 20 countries using an iterative Delphi process to ensure cross-cultural relevance and validity. While previous studies have looked at psychometric properties of the instrument, they have not interrogated the extent to which ISTOF is useful for evaluating individual lessons and providing feedback to teachers. In this study, we observed three grade 4 mathematics lessons taken from the NCTE video library at Harvard University for this purpose. Findings show that ISTOF can provide a highly differentiated and fine-grained picture of individual lessons, but that the strengths of the generic approach in terms of breadth are to an extent counterbalanced by limitations such as the lack of attention to content richness.