Bullying is aggressive behavior (words, actions, or social exclusion) that intentionally harms another person. Bullies typically choose victims based on a power imbalance that makes it difficult for victims to defend themselves. Bullying is usually persistent and repetitive. Verbal, physical, and emotional abuse is common in bullies' families, producing in bullies low self-esteem and a feeling of being unloved and/or insignificant. Male bullies tend to be physically mature and domineering. Victims tend to be physically weak and either overweight or underweight. They tend to have difficulty in relating to peers in general and have poor or ineffective social skills, making them less popular than others and excluded from peer groups. Bullying happens in all areas of the school. Prevention programs should involve attention to the emotional needs of all students, staff monitoring of all school spaces, the development of antibullying policies with as many stakeholders as possible, communication of the policy to students and parents, and programs that focus on constructive conflict resolution and assertiveness training. An important element of effective interventions is the elimination of "bystanding," i.e., the observation of bullying incidents without attempting to stop them. Although bullying behavior must be stopped, bullies themselves should be counseled in ways that address the causes of their behavior. Victims require psychological support and training in how to respond more constructively to bullying behavior. 10 references