Aggression is a behavior characterized by strong self-assertion in hostile or harmful tones. Under some circumstances, aggression may be a normal reaction to a threat. Alternatively, it may be abnormal, unprovoked, or reactive behavior (intermittent explosive disorder). Anger, confusion, discomfort, fear, overstimulation, and tiredness can lead to aggressive reactions.

Aggressive behaviors may be directed at oneself, at others, at animals, or at property. They can be verbal or physical. They can be premeditated and goal-oriented or impulsive. They can be direct or indirect, overt or covert.

Aggression is a potential symptom of diseases, disorders, or conditions that interfere with thought processes, such as brain tumours, dementia, post-traumatic stress disorder, schizophrenia, and a number of personality disorders. Although specific causes of aggression are not known, some studies have shown that abnormal brain chemistry or structural changes may play a role. Environment and genetics also seem to be involved.

Aggressive behaviors can lead to academic, employment, financial, legal, and relationship problems. Associated actions may result in incarceration or hospitalization. The success of rehabilitation and treatment depends on the underlying cause of the aggression.