This paper describes and analyses an in-service education project, financed by the Belgium Technical Cooperation, to improve the quality of mathematics teaching in 138 primary and lower secondary schools in Cambodia. The project design drew on recent research in developing countries and prior experience of training programmes in Cambodia. The research focused on the components of the in-service programme, which included direct instruction, classroom observation, training practice and self-evaluation. Data were collected from observations of the teacher trainers' participation in the project over a nine-month period. Additionally a sample of trainers kept diaries of their learning and were interviewed at regular intervals. The data collected were used to analyse which components the trainers perceived to make the greatest contribution to their learning. (Contains 3 figures.)