This thesis investigates bullying and victimisation in juvenile justice institutions. It examines the role of personal characteristics and aspects of institutional environments, and explores how these factors relate to bullying behaviour. The research collected quantitative and qualitative data using a mixed-method approach. The study comprised a survey completed by 289 male and female young people, aged 12 to 21 years old, in 8 juvenile justice institutions, using the scale version of Direct and Indirect Prisoner behaviour Checklist (DIPCSCALEDr) and Measuring the Quality of Prison life (MQPL) instruments. In addition, 24 interviews were carried out with 16 young people and 8 institutional staff, comprising both male and female participants. The findings showed that 95 percent of young people reported at least one behaviour indicative of bullying others, and 99 percent reported at least one behaviour indicative of victimisation in a month. The DIPC‐SCALEDr scored significantly higher on verbal forms of bullying and victimisation than psychological, physical, sexual, theft‐related and indirect forms. In addition, eight predictors are found to underpin bullying behaviour, including four personal characteristics i.e. time spent in the institution; experiences of punishment inside the institution; gang membership; and no self-harm ; and four institutional dimensions i.e respect; bureaucratic legitimacy; fairness; and family contact. In the interviews, young people and staff members identified four functions underpinning aggressive behaviour in the institution, including ‘protecting oneself from threatening events’, ‘exerting control over others’, ‘access to goods’ and ‘building alliances’.Furthermore, young people and staff members identified eight predictors that influenced their choices and decisions in bullying others. The strong desire to protect and enhance one’s sense of power and self-worth underpins illegitimate coping such as bullying others. In contrast, perceived negative outcomes of bullying conduct decreased an individual’s likelihood of bullying others. To conclude, bullying behaviour seemed to be normalized in juvenile justice institutions.