Fighting success and dominance in crayfish depends on a variety of extrinsic and intrinsic factors. Most intrinsic factors studied to date are related to the physical size of the crayfish. Larger crayfish or those with larger chelae often have advantages in agonistic interactions. Age, sex, and reproductive status can influence the physical size of the carapace and major chelae. These and other factors (e.g., social history) may also influence the intrinsic neurochemistry of individuals. Extrinsic factors that lead to increased dominance include the availability of resources, signal transmission properties, and status pheromones in the environment. Presently, it is largely unknown how these factors interact together to produce dominance. From an ultimate perspective, the evolutionary consequence of aggression and dominance is assumed to be tied to reproductive success or increased resource holding potential. This chapter uses a multidisciplinary and multilevel approach to address the question “what makes a crayfish dominant?”