Recent studies indicate that bullying is associated with adverse outcomes for both the victim and bully. Data from the Health Behaviour in School-aged Children Study provide a unique opportunity to compare bullying rates across 25 countries. Of 113 200 children surveyed, involvement in bullying varied dramatically from 9% to 54%. In all countries, bullying was associated with poorer psychosocial adjustment, greater health problems, and an increased risk of substance abuse. Bullying is a common, frequent, and important health issue for children globally.