Ecologists have invoked competition as one of the primary biotic factors that explains the distribution and abundance of many taxa. Species may compete indirectly for resources or directly through various types of interference competition. Examination of competitive interactions can occur in laboratory settings and in the field. The benefit of lab studies is that behaviors can be readily observed. Field experiments can assess competition for resources under natural levels of resource abundance. Both approaches have been applied to the study of competition in crayfish. Lab experiments have been used to assess levels of aggression and competition for shelter and food. Field experiments have been used to determine if crayfish affect resource abundance and hence survival and growth of other crayfish. We reviewed both lab and field studies of competition in crayfish. Almost every lab experiment concluded that crayfish compete aggressively for shelter or food. The opposite conclusion was reached by authors conducting field experiments; there was little evidence that competition affected crayfish survival or growth in field experiments. Despite the lack of support for competition by crayfish when examined in a field setting, competition is frequently invoked as an important factor affecting crayfish populations, particularly when one species is an invader. We urge caution when concluding that declines of crayfish are the result of competition. We also recommend that more field experiments evaluating competition between crayfish species be conducted.