Purpose. It has been argued, extensively and internationally, that sustained, school-based continuous professional development (CPD) has the potential to overcome some of the shortcomings of traditional one-off CPD programmes. Yet, the evidence-base on more- or less- effective forms of CPD is contradictory. The mechanisms by which sustained support should be offered are unclear and the impacts on teachers’ and students’ learning are complex and difficult to track. The purpose of this study was to examine the impact of a sustained, school-based, tailored and supported CPD program on teachers’ practices and students’ learning. Methods. Data is reported from six case studies of individual teachers engaged in a year-long CPD program focussed on Cooperative Learning. The CPD program involved participatory action research and frequent interaction/support from a boundary spanner (researcher/facilitator). Data were gathered from 29 video-recorded lessons, 108 interviews, and 35 field journal entries. Results. (a) Individualised (external) support, (b) departmental (internal) support, and (c) sustained support, impacted on teachers’ practices of Cooperative Learning. The teachers adapted their practices of Cooperative Learning in response to their students’ learning needs. Teachers began to develop a level of pedagogical fluency, and in doing so, teachers advanced students’ learning. Conclusions. Since this study demonstrates impact, it contributes to international literature on effective CPD. The key contribution is the detailed evidence about how and why CPD supported six individual teachers to learn - differently - and the complexity of the learning support required to engage in on-going curriculum development to positively impact student learning.