School bullying and cyberbullying is a widespread phenomenon among young people and it is used to describe interpersonal relationships characterized by an imbalance of power. Most empirical studies on school bullying and peer victimization are quantitative and examine the prevalence of bullying, the associated individual and personal factors, and negative outcomes. The present PhD thesis employed a mixed methods approach. The main goal of the quantitative study was to investigate the frequency of school bullying/victimization and cyberbullying/cybervictimization in secondary schools of Cyprus and Greece and how it relates to demographic, socio-academic and socio-emotional factors. The main goal of the qualitative study was to describe in more depth the phenomenon of school bullying/victimization to secondary school students in Crete and Cyprus. Participants were 800 adolescents from secondary school classes in Crete and Cyprus. In the qualitative study, 10 adolescents were interviewed. Self-report standardized scales and semistructured interviews were used. The research tools of quantitative part were the following scales: the Olweus Bully/Victim Questionnaire (OBVQ), the Cyberbullying Questionnaire (CQ), the Strengths and Difficulties Questionnaire (SDQ), the Resilience Adolescents Development Module (RYDM), the Bryant Empathy Index (BEI), the Parental Acceptance Rejection Questionnaire (PARQ), the Teacher Acceptance Rejection Questionnaire (TARQ) and self-construct semi-structured interview. The software packages that were used for data analysis and processing were SPSS 25.0 and the NVivo 7.0 for the thematic analysis of the semi-structured interviews. According to the results, the students as a whole experienced a low level of victimization in Greece and Cyprus. The most common forms were physical, verbal, and relational bullying. Most students from Crete were categorized as lowlevel victims. Respectively, fewer students from Cyprus were categorized as moderate and high-level victims. On the basis of both the quantitative and the qualitative data, findings suggest that few students were victimized through cyberbullying. Most students internalized their victimization experiences and turned to their parents for help, but their friends were the reference persons. The number of good friends in the classroom, the degree of school liking seems to be related to the degree of school bullying and victimization. The students in the sample were characterized by high levels of strength, resilience and acceptance from parents and low levels of understanding feelings of others. As children became older it was less likely to report instances of being victimized by others. The more children reported to like school, the less likely it was to report instances of having been bullied from others. The Cretan adolescents reported less instances of being victimized and bullied from others. The predictors of school victimization were region, age 13 or 15, no friends, resilience total, acceptance total and difficulties, while for school bullying were region, don’t liking school, 1 or 2-3 good friends, resilience total and acceptance total. The predictors of cyberbullying were region, age 15, like school and like very much, while for cybervictimization were region, age 15 and like very much. Overall, the study provides evidence that employing a mixed methodology to study school bullying/victimization and cyberbullying/victimization in secondary schools is very useful. Such approach can provide insights in further understanding the phenomenon in depth, which has implications for the development of appropriate prevention and intervention programmes to address the issue.