Fishes, particularly parrotfishes (Scaridae) and surgeonfishes (Acanthuridae) and the regular sea urchins (Echinoidea) are abundant, conspicuous, and in some areas commercially important consumers of algae and seagrasses in the Caribbean. Diet-food availability studies show that herbivores generally take food in proportion to its abundance and availability in the field. These studies combined with direct food preference tests indicate that some plants or parts of plants are avoided by herbivores. The possibilities of chemical or structural factors which render plants unpalatable are being investigated. Herbivores attain large population densities and their feeding activities greatly influence plant distributions and community structure. Dense algal growth generally occurs only in areas inaccessible to herbivores such as wave-washed reef surfaces or beachrock benches. Herbivores are also responsible for the halo region of sparse and grazed seagrasses separating reefs from surrounding seagrass beds. Experimental manipulations of herbivores on reefs lead to rapid changes in the structure of plant communities.