This paper reviews the current concepts of recognition of non-self in crustaceans and relates these concepts to recognition in the invertebrates in general. It focuses primarily on a decapod crustacean, the blue crab(callinectes sapidus)and on results using this animal as a model to study the clearance of viruses and xenogeneic proteins. Clearance studies indicate that blue crabs possess a quasi-specific recognition system in the normal or “non-immunized”state. This system is capable of rapidly clearing foreign proteins and certain viruses from the circulation, and results in concentration of such proteins into the gills and viruses into the hepatopancreas or gills. Although humoral factors which bind foreign proteins or neutralize viruses have been isolated, transfusion exchange experiments involving depletion of circulating cells and/or humoral factors indicate that the blue crab does not require circulating hemocytes or humoral factors for clearance of foreign proteins. These results suggest that a population of fixed cells, possibly those in the gills, may be the critical component for recognition of foreign proteins by normal crabs.