The ecology of nemertean predators in marine ecosystems is reviewed. Nemerteans occur in most marine environments although usually in low abundances. Some species, particularly in intertidal habitats, may reach locally high densities. During specific time periods appropriate for hunting, nemerteans roam about in search of prey. Upon receiving a stimulus (usually chemical cues), many nemertean species actively pursue their prey and follow them into their dwellings or in their tracks. Other species (many hoplonemerteans) adopt a sit-and-wait strategy, awaiting prey items in strategic locations. Nemerteans possess potent neurotoxins, killing even highly mobile prey species within a few seconds and within the activity range of its attacker. Most nemertean species prey on live marine invertebrates, but some also gather on recently dead organisms to feed on them. Heteronemerteans preferentially feed on polychaetes, while most hoplonemerteans prey on small crustaceans. The species examined to date show strong preferences for selected prey species, but will attack a variety of alternative prey organisms when deprived of their favourite species. Ontogenetic changes in prey selection appear to occur, but no further information about e.g. size selection is available. Feeding rates as revealed from short-term laboratory experiments range on the order of 1-5 prey items d-1. These values apparently are overestimates, since long-term experiments report substantially lower values (0.05 - 0.3 prey items d-1). Nemerteans have been reported to exert a strong impact on the population size of their prey organisms through their predation activity. Considering low predation rates, these effects may primarily be a result of indirect and additive interactions. We propose future investigations on these interactive effects in combination with other predators. Another main avenue of nemertean ecological research appears to be their role in highly structured habitats such as intertidal rocky shore habitats and coral reef environments.