This paper highlights insights into the processes and effects of an in-service training on "School Policies on Special Needs Education" that we ran at the Centre for Educational Policy and Innovation, University of Leuven. Our training was based on the explicit concept of professional development as a career-long process of learning and development, resulting from the meaningful interaction between the "learner" and the context in which s/he works (Kelchtermans & Hamilton, 2004). This theoretical framework affects the design, content and organization of the training. Our concept of professional development implies that the participants' own meaningful career experiences and their professional contexts, are central to conceiving of in-service activities. Consequently, if we want in-service training to be effective and to contribute to professional development and school development, particular attention needs to be paid to the permanent interaction of the individual participant (and his/her "characteristics"), his/her professional context and the position of the in-service training in this interaction (also see Ballet, et al., 2000). The effectiveness of in-service training depends on the way in which these elements are tuned. By carefully mapping and interpreting this interaction as part of the ongoing INSET project, we disentangle the complexity of in-service training.