Objective: To determine whether the relationship between bullying and psychosocial adjustment is consistent across countries by standard measures and methods. Design: Cross-sectional self-report surveys were obtained from nationally representative samples of students in 25 countries. Involvement in bullying, as bully, victim, or both bully and victim, was assessed. Setting: Surveys were conducted at public and private schools throughout the participating countries. Participants: Participants included all consenting students in sampled classrooms, for a total of 113 200 students at average ages of 11.5, 13.5, and 15.5 years. Main Outcome Measures: Psychosocial adjustment dimensions assessed included health problems, emotional adjustment, school adjustment, relationships with classmates, alcohol use, and weapon carrying. Results: Involvement in bullying varied dramatically across countries, ranging from 9% to 54% of youth. However, across all countries, involvement in bullying was associated with poorer psychosocial adjustment (P<.05). in all or nearly all countries, bullies, victims, and bully-victims reported greater health problems and poorer emotional and social adjustment. Victims and bully-victims consistently reported poorer relationships with classmates, whereas bullies and bully-victims reported greater alcohol use and weapon carrying. Conclusions: The association of bullying with poorer psychosocial adjustment is remarkably similar across countries. Bullying is a critical issue for the health of youth internationally.