‘Grazing ecosystem’ is typically used to describe terrestrial ecosystems with high densities of mammalian herbivores such as the Serengeti in East Africa or the Greater Yellowstone Ecosystem in North America. These abundant, large herbivores determine plant community dynamics and ecosystem processes. The general concepts that define grazing ecosystems also aptly describe many aquatic ecosystems, including coral reefs, seagrass beds, and lakes, where herbivores such as parrotfishes, turtles, and zooplankton have strong impacts on ecosystem processes. Here, I compare the ecology of grazing ecosystems in search of common concepts that transcend the terrestrial-aquatic boundary. Specifically, I evaluate: 1) the feedbacks between herbivory and primary production, 2) the roles of herbivore richness and facilitation, 3) how predators and diet quality shape patterns of herbivory, and 4) how altering herbivory mediates alternative states.