PS is multi-component intervention at the primary prevention level and aims at creating a positive learning environment and decreasing problem behaviours among students in the school years 4-9. During 2009-2012 the programme was both implemented and studied for the first time in a Swedish context. The present thesis comprises four studies of PS and applies a mixed methods approach. The thesis aims to study: 1) If the programme is effective in enhancing the classroom climate, and decreasing problem behaviours among students (Study I), 2) teachers and school staff’s perceptions of programme and implementation (study II and III) and 3) if teacher’s use of praise and clarity of school rules, regardless of programme use, are useful as classroom management techniques, and if there are any differences between classes in terms of disruption (study IV). A quasi-randomized trial was conducted in study I, comprising 3207 students in school grades 5-7 and 188 teachers in 23 schools in the wider area of Stockholm. The participating children and their head teachers answered self-report questionnaires on three occasions: At baseline and one and two years after programme initiation. Degree of implementation in the schools was also measured. Further, a qualitative design with semi-structured interviews and thematic content analysis was used in study II and III. The interviews were conducted in seven PS schools with seven school leaders and 13 school teachers during 2010 and 2011. In study IV, students (n=2266) and classes were divided into two groups, based on teacher ratings of disruptive students in class. Baseline and 12-month follow-up responses were used to perform multiple regression analysis, to compare groups and to investigate possible longitudinal associations. Study I showed no significant effects on students’ problem behaviours and classroom climate at last follow-up. The findings from study II indicated barriers to programme commitment in terms of lack of consensus, collaboration barriers and insufficient process management. It was concluded that leadership, coaching and staff selection need particular attention when implementing a programme like PS, since those factors have been defined as important implementation drivers, both in this study and previously. Study III showed that teachers’ professional identity, programme understanding and experience of change were factors affecting implementation. Ambiguities regarding the boundaries of the social assignment, opposition against the theoretical underpinnings and an unclear core component were identified as implementation barriers. In study IV it was shown that clarity of school rules did not substantially contribute to classroom climate, whereas teacher’s use of praise to some extent did. The hypothesis on weaker associations in the low disruption group could not be confirmed. This thesis cannot provide an answer to the question whether PS is effective or not, since findings indicate that the implementation did not succeed. However, findings in the last study indicate that teacher’s use of praise may contribute to improve the classroom climate. If the PS programme is to be used again there is a need for revisions, and sufficient time for schools to consider programme adoption will be crucial. Aspects of programme implementation are further discussed.