This PhD thesis is a part of the project 1 "The social and moral processes of bullying -a four-year longitudinal study," funded by the Swedish Research Council (Grant number 2013-07753). The project was carried out between the academic years 2015/16 and 2019/20, and entailed that participants answered a web-based questionnaire once each year. At the start of the project all students attended fourth grade and at the end of the project they attended eighth grade. This thesis used data from the first four waves of datacollection, along with data from a pilot study carried out in the spring of 2015, with students in grades 4 to 8.This thesis adopted Bandura's (1986Bandura's ( , 2016 social cognitive theory as an overarching theoretical framework for understanding school bullying in early adolescence. According to social cognitive theory, bullying and bystander behaviors cannot be reduced only to individual variables, but have to be understood as results of a complex interplay between individual and environmental factors.The overall objective of the project was to examine bullying, victimization, and bystander behaviors among school children in relation to social, cognitive, and moral factors -concurrently and over time. However, in this thesis I have examined only a limited part of the overall objective, more specifically the association between moral disengagement and bullying in early adolescence.