Astacoidean crayfishes serve as hosts to obligate ectosymbiotic annelids called branchiobdellidans. Branchiobdellidans can act either as mutualistic cleaners or as ectoparasites and can have strong effects on crayfish growth and survivorship. This potentially vital aspect of crayfish biology has gone largely unexplored until recently. We reviewed the current state of knowledge regarding this symbiosis and examined factors that contribute to variability in the effects of branchiobdellidans on crayfish. We show that branchiobdellidans affect crayfish in various ways depending on branchiobdellidan species, abundance, and ecological context. We also discuss evidence for regulatory controls that crayfish exert over their symbionts and symbiont–host preferences. Last, we evaluate the utility and challenges of using the crayfish–branchiobdellidan association as a model system for ecological and evolutionary research and point to promising areas for future study. Further investigations of the complex interactions between crayfish and their ectosymbionts will greatly advance the field of crayfish biology and offer many exciting opportunities for the study of symbioses.