Recent studies suggest that insects use pattern recognition molecules to distinguish prokaryotic pathogens and fungi from “self” structures. Less understood is how the innate immune system of insects recognizes endoparasitic Hymenoptera and other eukaryotic invaders as foreign. Here we discuss candidate recognition factors and the strategies used by parasitoids to overcome host defense responses. We suggest that host–parasitoid systems are important experimental models for studying how the innate immune system of insects recognizes foreign invaders that are phylogenetically more closely related (...) to their hosts. The strategies used by parasitoids suggest that insects may employ “hidden-self” recognition molecules for attacking foreign objects intruding the open circulatory system. BioEssays 23:344–351, 2001. © 2001 John Wiley & Sons, Inc. (shrink)