Symbionts are well known to have benefi cial and/or pathogenic effects on crayfi sh health, and as such assert direct impacts on freshwater ecosystems and commercial astaciculture. Several reviews (e.g., Vogt 1999, Evans and Edgerton 2002, Edgerton et al. 2002, Longshaw 2011, citations therein) provide valuable resources of information on crayfi sh symbionts ranging from viruses to arthropods. Although a useful starting point for interested researchers, these reviews may no longer accurately refl ect many recent shifts in our understanding of branchiobdellidan taxonomy, biology and hostsymbiont interactions. Branchiobdellidans, or crayfi sh worms (Fig. 23.1), comprise an order in which all known species are obligate ectosymbionts of freshwater crustaceans, primarily astacoidean crayfi shes. No free-living members are known (although see Holt 1973a, Timm 1991). Consequently, our understanding of branchiobdellidan ecology and evolution is inextricably intertwined with that of their crayfi sh hosts. Recent studies on branchiobdellidans have illuminated several errors and inaccuracies in earlier works. Consequently, our aim for this chapter is to provide an overview of current knowledge of branchiobdellidans, with a particular focus on aspects that are of interest to astacologists. For additional detail we direct readers to Gelder and Williams (2015).