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.