Animals commonly modify their behavior in the presence of a conspecific or in response to signals. This is particularly true in the context of aggressive exchanges, which animals use to form networks of social relationships and to communicate social status associated with those relationships. Although hierarchical structures are a widespread phenomenon that has been studied extensively, the dynamic communication processes, specifically chemical communication in this review, is relatively overlooked. In particular, it is the exchange of information during agonistic interactions that mediates hierarchies and/or alters the outcomes of agonistic interactions. Given the theoretical appeal of these interactions, and the evolutionary importance and taxonomic diversity associated with social hierarchies, it is not surprising that the sensory mechanisms involved in the formation and maintenance of hierarchical structures have received recent attention. In crayfish, dominance is thought to be largely determined by physical superiority, where encounters are largely dyadic and fighting behavior is highly stereotyped. However, recent evidence has shown that the outcome of dyadic encounters are dependent upon a number of factors other than physical size, that include the exchange of chemical information during encounters, previous social history, and the intrinsic neurochemical state of opponents. We have attempted to provide a comprehensive analysis of the extrinsic chemical processes (previous history, sensory communication, etc.) and intrinsic chemical processes (neurochemical state) that produce and maintain dominance relations and social hierarchies in crayfish. We hope that this review will bring together a global picture of the processes that determine a crayfish's social standing and how intrinsic and extrinsic chemicals have substantial effects on aggressive states and agonistic bouts.