This article describes an in-service teachers', staff-development model "Peer Learning Network" and presents results about its efficiency. "Peer Learning Network" promotes three levels of peer learning simultaneously (among pupils, teachers, and schools). It supports pairs of teachers from several schools, who are linked through a network, to use cooperative learning in their classrooms by implementing an adjustable peer-tutoring programme. The programme offers evidence of the progress of the pupils, and tries to guarantee the sustainability through a three-year, staff-development sequence that replaces the teacher pairs each year. After describing the peer-tutoring programme, results from 20 schools from one network in Spain are presented. These results show the effects on teacher learning on both concepts and attitudes, the decisions that pairs of teachers make to adjust the programme to their context, and the supports needed for success. This programme has been shown to be an effective and efficient way to help teachers introduce and use, in a sustained way, a method of cooperative learning in their classrooms.