Research over the past two decades has demonstrated the effectiveness of co-operative learning (CL) methods for the promotion of pupil learning and social relations. CL methods are not commonly used in Dutch primary schools where learning is considered primarily an individualistic enterprise. At this moment, primary school teachers receive little or no professional training for promoting CL methods in their classrooms. Therefore, a staff development program was developed aimed at providing teachers with instructional competencies that could assist them in using CL methods at the classroom and school level. The key elements of the program are: (1) schools participating in the implementation study will receive training, materials, coaching and follow-up assistance for two or three consecutive years; (2) in order to implement long-term use of CL, certain school environment elements will also be considered, such as peer coaching, teacher collaboration, collaboration between principal and teachers, parent involvement, and cooperation between schools; (3) from simple to complex: first training in relative simple structured CL methods as developed by Kagan (1994), then training in more complex methods based on the work of Slavin (1995) and Johnson and Johnson (1994); (4) CL methods will be connected with the well-known direct instruction model; (5) the staff development program will be based on a constructivist conception of learning. During implementation, achievement tests will be administered to evaluate the program; also data on the social relations between pupils will be collected. Descriptive data on the use of CL methods by teachers and pupils will be collected by observations and interviews.