Abstract Symbiotic associations have arisen independently in several groups of the largely free-living turbellarians. Morphological adaptations of turbellarians to a symbiotic way of life include suckers and adhesive glands for attachment, elaborate systems of microvilli and other epidermal structures for absorption of food, glands for the formation of cysts, cocoons and cement material, and lack of a pharynx and intestine in some species. However, many species closely resemble their free-living relatives. Egg production is greatly increased at least in some species, and life cycles are always direct. Food of symbiotic turbellarians consists of host food and/or host tissue. Ectosymbiotes show fewer physiological adaptations than entosymbiotes. The major groups of parasitic Platyhehninthes (Trematoda Aspidogastrea, Trematoda Digenea, Monogenea, Udonellidea, Cestoda including Gyrocotylidea, Amphilinidea and Eucestoda), form one monophylum, the Neodermata, characterized by a neodermis (tegument) replacing the larval epidermis, epidermal cilia with a single horizontal rootlet, sensory receptors with electron-dense collars, spermatozoa with axonemes incorporated in the sperm body by proximodistal fusion, and protonephridial flame bulbs formed by two cells each contributing a row of longitudinal ribs to the filtration apparatus. The sister group of the Neodermata is unknown but is likely to be a large taxon including the Proseriata and some other turbellarian groups. Among the Neodennata, the Aspidogastrea is likely to be the most archaic group, as indicated by DNA studies, morphology, life cycles and physiology. Aspidogastreans can survive for many days or even weeks outside a host in simple media, they show little host specificity, and have an astonishingly complex nervous system and many types of sensory receptors, both in the larva and the adult. It is suggested that Aspidogastrea were originally parasites of mlluscs (and possibly arthropods and other invertebrates) and that they are archaic forms which have remained at a stage where vertebrates represent facultative hosts or obligatory final hosts into which only the very last stages of the life cycle (maturation of the gonads) have been transferred. The complex life cycles of Digenea have evolved from the simple aspidogastrean ones by intercalation of multiplicative larval stages (sporocysts, rediae) in the mollusc host, and of cercarial stages ensuring dispersal to the now obligatory final host. Monogenea may have lost the molluscan host or evolved before the early neodermatans had acquired it. Cestoda either replaced the original molluscan with an arthropod host, retained an original arthropod host or evolved from an early neodermatan before molluscan hosts had been acquired, newly acquiring an arthropod host. Horizontal gene transfer and implications for mosaic evolution in the Platyhehninthes are discussed.