This is a study of schools as professional learning communities, defined by nine characteristics and their relationship with the schools' level of effectiveness. The study was conducted within three schools in Iceland. It was designed as a mixed methods study, conducted in two phases: a correlational study of survey data on schools as professional learning communities and an experimental study, where effort was made to improve the level of the professional learning community and evaluate its effects on pupils' outcomes. In both phases relatively strong evidence was obtained on the relationship between a school's level of effectiveness and its level as a professional learning community. In the experimental school, interventions resulted in higher scores on national tests, especially in mathematics. (Contains 7 tables and 1 figure.)