In this paper, I consider the importance of the work of Bert Creemers to my own work and the field of educational effectiveness. In particular, the field needs to take heed of 3 key lessons from Bert Creemers' work: the importance of an empirical and sceptical stance focussed on what actually works in education, the primacy of the classroom level in educational effectiveness, and the need for theory development in educational effectiveness and educational research more generally. I also discuss 3 challenges to the field: the need for equity as well as effectiveness, the need for the field to look beyond the school, and the need to remain relevant in a changing educational landscape.