There is an important relationship between the immune system and aggressive behavior. Aggressive encounters acutely increase the levels of proinflammatory cytokines, and there are positive correlations between aggressive traits and peripheral proinflammatory cytokines. Endotoxin lipopolysaccharide (LPS) treatment, which results in peripheral immune activation, decreases aggressive behavior as one of the sickness behavioral symptoms. In contrast, certain brain infections and chronic interferon treatment are associated with increased aggression. Indeed, the effects of proinflammatory cytokines on the brain in aggressive behavior are bidirectional, depending on the type and dose of cytokine, target brain region, and type of aggression. Some studies have suggested that microglial activation and neuroinflammation influence intermale aggression in rodent models. In addition, pathological conditions as well as physiological levels of cytokines produced by microglia play an important role in social and aggressive behavior in adult animals. Furthermore, microglial function in early development is necessary for the establishment of the social brain and the expression of juvenile social behaviors, including play fighting. Overall, this review discusses the important link between the immune system and aggressive traits and the role of microglia as mediators of this link. This article is part of the Special Issue on "Personality Disorders". Heterogeneity of cytokine/microglial involvement in aggressive behavior. CC: cingulate cortex, HIP: hippocampus, DRN: dorsal raphe nucleus, PAG: periaqueductal grey, MH: medial hypothalamus. Display omitted •Proinflammatory cytokines have bidirectional effects in aggressive behavior.•Both neuroinflammation and physiological levels of cytokines produced by microglia influence aggressive behavior.•Microglial function during early development is necessary for juvenile social behavior including play fighting.