Aggressive behavior is defined as social behavior carried out with the intention to harm. Violence denotes those forms of aggression that are intended to cause severe physical harm. Aggressive behavior has severe negative consequences for individuals, social groups, and societies as a whole. Therefore, understanding why some individuals are more prone to engaging in aggressive behavior than others and some situational circumstances and social contexts are more likely to elicit aggressive behavior is a critical task. Influential psychological theories of aggression conceptualize aggression as the result of the interplay between variables in the person and the situation. To explain individual differences in aggressive behavior, one line of research has looked at broad personality dimensions, such as self-esteem and narcissism, lack of self-control, and the “Big-Five” personality factors. Evidence shows that high narcissism, low self-control, low openness to experience, conscientiousness, extraversion, and agreeableness, and high neuroticism are linked to a higher propensity to engage in aggressive behavior. Another line of research has focused on more circumscribed, aggression-related personality constructs, demonstrating that individuals who are habitually anger-prone, have a tendency ruminate about anger-eliciting experiences, and show a hostile attributional style in terms of seeing other persons’ behavior as an expression of hostile intent are more likely to show aggressive behavior. On the side of the situation and social environment, several conditions have been identified under which the likelihood of aggressive behavior is increased. Individuals are more likely to show aggressive behavior when they have consumed alcohol, after they have experienced social rejection by others, when aggressive cues, such as weapons, are present in the situation, and when they have access to a firearm. Aggression is also more likely to be shown under conditions of anonymity and high temperature and as a result of regular exposure to depictions of violence in the media. In addition to such “main effects,” there is evidence of an interactive effect of individual and situational characteristics. For example, the impact of exposure to violent media is greater on individuals with a higher disposition to show aggressive behavior, and the effect of alcohol consumption on aggression is greater among people who are habitually prone to engage in angry rumination. Approaches to preventing aggression may build on the evidence on personal and situational differences. For example, anger management trainings may promote better control of angry impulses, focusing on the personal risk factors for aggression, whereas providing role models who show nonaggressive responses in anger-eliciting situations reflects a focus on situational interventions. In conclusion, personality and situational variables need to be considered in combination and interaction to predict when aggressive behavior is likely to occur. Gaining a better understanding of the factors promoting aggressive behavior needs to remain high on the agenda for theory building and empirical research in psychology.