The aim of this thesis is to explore methods for assessing peer victimization and pedagogical leadership in school. The thesis includes four studies. Study I and II are based on web-based questionnaires among 2, 568 students in grades 7, 8 and 9. Study III is based on a questionnaire (n=128) and four focus group interviews (n=21) among students in grades 7 and 9. Study IV is based on a web-based questionnaire including 344 teachers. The results from Study I showed that among students who experienced peer victimization 13% were captured by a bullying measure, 44% by a measure of repeated peer aggression, and 43% by both measures, i.e. the two measures captured partly different pupils. Study II showed that the two measures captured the same proportion of adolescents with psychosomatic problems and showed no significant differences in mean values on the Psychosomatic Problems (PSP) scale. In Study III it was shown that besides the traditional criteria the adolescents definition of bullying also included a criterion based on the health consequences of bullying. That is, a single but hurtful or harmful incident could also be considered bullying irrespective of whether the traditional criteria were fulfilled or not. The Rasch analysis in Study IV indicated two sub dimensions of the Pedagogical and Social Climate (PESOC-PLP) scale; direct pedagogical leadership and indirect pedagogical leadership. Satisfying psychometric properties indicated that the PESOC-PLP scale could be used to measure pedagogical leadership of the principal. This thesis highlights problems with how bullying and school leadership is currently defined and measured. By strengthening the understanding of measurement methods of peer victimization and school leadership the aim is that the results from this thesis will contribute in providing a safe and positive school experience for children and adolescence and that it can be used as a valuable tool to combat peer victimization. / Baksidestext: The negative consequences of peer victimization on children and adolescents such as worsening academic achievement and mental ill health are major public health concerns which have been subjected to extensive research. However, there are long-standing concerns how to define, measure, and estimate prevalence rates of peer victimization and successful school leadership. The aim of this thesis is to study methods for assessing peer victimization and pedagogical leadership in school. The results show that excluding other forms of peer victimization than bullying have serious implications for the identification of victims and may underestimate the full impact of peer victimization on children. Further, the validation of the Pedagogical and Social Climate (PESOC-PLP) scale is a step towards ensuring valid assessments of pedagogical school leadership. By strengthening the understanding of measurement methods of peer victimization and school leadership the aim is that the results from this thesis will contribute in providing a safe and positive school experience for children and adolescence and that it can be used as a valuable tool to combat peer victimization.