Over their 500 million yearhistory, gastropods have radiated into marine, freshwater and terrestrial environments and adopted life styles ranging from herbivory to carnivory to endoparasitism to symbiont-mediated chemoautotrophy. They contend with many pathogens, including several lineages of specialized eukaryotic parasites. Their immunobiology is as yet poorly known, in part because most studies focus on a very small segment of gastropod diversity. Gastropod genome sequences are now forthcoming but synthetic overviews of the gastropod immunome are not yet available. Most immunological studies focus on interactions between gastropods and the larval stages of digenetic trematodes (digeneans) such as the medically important schistosomes. Digeneans elicit demonstrable and relevant snail defense responses and provide insights, augmented by the recently available schistosome genome sequences, for how gastropod responses are subverted. Survival of digeneans in snails depends at least in part on their ability to mimic host glycotopes, to overcome the immediate attack ofreactive oxygen and nitrogen species produced by host hemocytes, and to induce long-term down-regulation of immune functions. Gastropods can mount distinct responses to different categories of pathogens, and can orchestrate effective elevated secondary responses under certain circumstances. Defense responses of at least one gastropod species, Biomphalaria glabrata, involve hemolymph lectins that are diversified by a variety of processes, including somatic diversification. Such observations have played a role in revising our general concept of invertebrate defense to include the possibility of more sophisticated and diversified responses beyond the production of limited repertoires of invariant pattern recognition molecules. The study of gastropod immunobiology is thus of basic interest and has several applied uses as well, including our need to conserve imperiled gastropod diversity.