Aggression is behavior with the intent to cause harm to others who are motivated to avoid that harm. It is part of the human condition—evident for most early in life, and for some, persistent across the lifespan. Aggression can take a variety of forms, from the subtlety of shunning to the extremity of violence. Aggression creates physical and mental health problems in targets and is associated with impairment and competence in perpetrators, depending on the form it takes. Aggression also causes problems at the collective level. It can destroy families, communities, religions, and nations. There is a large body of research documenting the etiology, developmental course, and consequences of aggression, with particular focus on understanding key factors, mechanisms, and functions of various forms of aggression. This research highlights that aggression is ubiquitous, transcends culture, and represents a notable barrier to society. However, with quality evidence-based interventions, aggression can be effectively reduced.