Teaching for the development of students' thinking is not a straightforward matter. It requires pedagogical skills, which are different from those of normal good quality teaching for conceptual development. It follows that providing professional development (PD) for teachers of thinking is a "hard case"--we can learn much of general value to providers and clients of PD from the specific case of a PD programme for teachers of thinking. A series of empirical studies are summarised and, combined with 30 years worth of first hand experience and a thorough review of the literature, are used to build a model of the factors which have a direct impact on the effectiveness of PD, that effectiveness being defined in terms of measurable changes in students. A fuller account of the work summarised in this paper is available in Adey et al. [Adey, P., Hewitt, G., Hewitt, J., & Landau, N. (2004). "The professional development of teachers: Practice and theory". Dordrecht: Kluwer Academic]. (Contains 3 figures and 1 table.)