The question “What are aggression and violence?” can be answered in many ways given the many forms that both phenomena can take. This chapter defines aggression and violence and provides a detailed discussion of distinctions that have emerged between different types of aggression. For example, aggression can be classified as physical, verbal, or relational; hostile or instrumental; reactive or proactive; impulsive or premeditated; direct or indirect; active or passive; overt or covert; legitimate or illegitimate; displaced, triggered displaced, or not displaced; and person based, situation based, or both person and situation based. Aggression is also differentiated from similar but distinct concepts including antisocial behavior, juvenile delinquency, coercion, assertiveness, aggressive cognition, and aggressive affect. This chapter provides readers with a clear understanding of what aggression and violence are as well as what they are not.