Aggression is an innate behavior that has likely evolved in the framework of defending or obtaining resources. This complex social behavior is influenced by genetic, hormonal, and environmental factors. In many organisms, aggression is critical to survival, but the ability to control and suppress aggression in distinct contexts also is necessary. Invertebrate organisms, with their relatively simple nervous systems and a multiplicity of powerful tools available to examine their often elaborate and complex behavioral displays, have become increasingly valuable models for investigating the genetic and systems biological roots of social behavior. In this protocol, we outline methods for analyzing aggression in Drosophila: The design encompasses eco-ethological constraints that emphasize an understanding of normal aggression. The details include steps for constructing a fight arena, isolating and painting flies, introducing flies to an arena, and videotaping and scoring fights. These experimental protocols are in current use to identify candidate genes important in aggression and to elaborate the neuronal circuitry underlying the display of aggression and other social behaviors.