With recent advances in pharmacology and genetic manipulation techniques, new interest has developed in the biological mechanisms of both non-human and human aggression. Although aggression is certainly a complex social behavior with multiple causes, molecular biological factors should not be overlooked, as they may well lead to interventions that prevent excess aggressive behaviors. The primary goal of this book is to summarize and synthesize recent advances in the biological study of aggression. As most aggressive encounters among human and non-human animals represent a male proclivity, the research in this book describes and discusses studies using the most appropriate murine model: testosterone-dependent offensive inter-male aggression, which is typically measured in resident-intruder or isolation-induced aggression tests. The research also emphasizes various molecules that have been linked to aggression tests. Although the evidence continues to point to androgens and serotonin (5-HT) as major hormonal and neurotransmitter factors in aggressive behavior, recent work with GABA, dopamine, vasopressin, and other factors, such as nitric oxide, has revealed significant interactions with the neural circuitry underlying aggression. This book is organized according to levels of analysis. The first section examines the genetic contributions to aggression in species ranging from crustaceans to humans. The section summarizes the involvement of various neurotransmitters and neuromodulators in aggressive behavior. The third section summarizes the influence of hormones on aggression, primarily in humans. All chapters emphasize future directions for research on aggression and reveal important domains that have received comparatively less attention in this literature.