This study reports on an investigation into the impact of a national, school-based teacher development programme on learning and teaching in Kenyan primary schools. Building on a national baseline study (n=102), 144 video-recorded lessons, covering the teaching of English, maths and science at Standards 3 and 6, were analysed to investigate whole-class teaching and group-based learning. Interviews were also conducted with school management committees, head teachers, teachers and pupils to elicit their views on the impact of the school-based training programme on learning and teaching. The study found that compared to the earlier baseline, teachers were more interactive with the pupils in their whole-class teaching and greater use was being made of group work. Lesson plans, teaching resources and flexible classroom layouts were also much more in evidence. However, the greatest impact on classroom practice was seen in the classrooms of those teachers who had undergone the most systematic in-service training. The wider implications of the findings for improving the quality of classroom learning in Kenyan primary classrooms are considered. (Contains 5 tables, 4 figures, and 1 note.)