How can we really evaluate teacher effectiveness? Systems of teacher appraisal and evaluation are being created across the world in order to monitor and assess teacher performance. But do the models used really give a fair evaluation? Based on international research, the authors argue that teacher effectiveness is too narrowly conceptualised and methods of measuring it are not attuned to the real contexts in which teachers work. They propose a model of differential teacher effectiveness which takes into account that: * teachers may be more effective with some categories of students than with others* teachers may be more effective with some teaching contexts than others* teachers may be more effective with some subjects or components than with others. Building on and developing previous research on models of teacher effectiveness and current theories, the authors open up possible new debates which will be of interest to academics and researchers working in this area throughout the world.