13

At school, bullying is one of the most common expressions of violence among peers. Research on bullying started more than 40 years ago, when the phenomenon was defined as aggressive, intentional acts carried out by a group or an individual repeatedly and over time against a victim who cannot easily defend themselves. Aggressive behavior that has the following three characteristics is defined as bullying: (1) repetition, (2) intentionality, and (3) imbalance of power. Prevalence, age, and gender differences are discussed, as well as why it happens, drawing first on the large surveys carried out in western countries. Risk factors and short- and long-term outcomes of bullying and victimization are considered. Finally, effective prevention is discussed and avenues for research proposed.