Pulmonate and prosobranch snails, being necessary hosts for parthenogenetic generations of digenetic trematodes, participate in transmission of all trematodoses important from medical point of view. Role of particular gastropods in epidemiology of these diseases is discussed in details. Invasion of land snails and slugs is mainly passive by eggs containing developed miracidia, while enter of these larvae into snails inhabiting fresh-water environments is usually active. Generations in the snail host between miracidia and cercariae vary considerable, depending upon the fluke species. Generally, the cercaria is produced by the sporocyst or the redia. Cercariae usually actively penetrate out of the infected snail and enter water. Leaving out of account cases of encystation of cercariae in external environment (liver flukes of the family Fasciolidae) and active penetration into the final host (blood flukes of the family Schistosomatidae), encystation of these larvae takes place in the second intermediate host. Cercariae of medically important flukes may develop into metacercariae in tissues of fish (liver flukes - Opisthorchis felineus, Clonorchis sinensis, intestinal flukes--Heterophyes heterophyes, Metagonimus yokogawai, Troglotrema salmincola), as well as in crustaceans (pulmonary flukes of the genus Paragonimus), insects (the bipathogenic liver fluke Dicrocoelium dendriticum and other species of the family Plagiorchiidae incidentally found in man), and gastropods (flukes of the family Echinostomatidae, among them perhaps the best known is the Oriental species--Echinostoma ilocanum). In prevention and control of human trematodoses, especially in endemic foci, elimination of snail hosts is a great importance. Using molluscides may be objectionable from the stadpoint of environmental modification through their toxicity to other organisms. Biological control of snail hosts is more attractive. It includes introduction and management of predators, parasites, or pathogens, and intramolluscan competition. Moreover, certain slugs and terrestrial snails participate in transmission of nematode larval stages, including species known as pathogenic for man - the strongylid nematodes of the genus Angiostrongylus). Such infections may be prevented by abstanding from eating raw or inadequately cooked molluscs in endemic areas.