Littorinid snails are predominantly herbivorous and the versatility of their radulae enables them to feed on a variety of macroscopic and microscopic plants in a diversity of habitats. Some are selective feeders preferring some species of algae to others, and rejecting some even after a prolonged period of starvation. Different species of snail exhibit different preferences. The factors affecting the attractiveness and edibility of food plants are discussed and food value considered. Foraging behaviour of littorinids is briefly reviewed in relation to the influence of chemical cues from the algae. Littorinids appear to be able to select or reject algae without having ingested them, having perceived the plants from a distance, moving towards favoured foods (or habitat-providing plants) and away from those that it rejects. The nature of the chemical cues emitted by the algae is discussed. Temporal patterns of foraging activity show some evidence of an endogenous component which can be overridden by responses to environmental conditions. These patterns place restraints on energy intake. The structural and chemical defences used by algae against littorinid grazing are considered. The importance of polyphenolic compounds is evaluated. The effects of grazing as a selective agency and a factor influencing algal populations are discussed. There is some evidence that life history patterns are a response to grazing. The influence of external physical factors, such as salinity on grazing pressure is demonstrated. Finally, the impact of littorinid snails on intertidal communities is assessed in relation to their abundance and biogeographical distribution. The relative importance of littorinids is contrasted on shores possessing or lacking limpets.