Grazing ecosystems are typified by high densities of herbivores and high rates of herbivory.  In these ecosystems, intense herbivory selects for fast growing plants that yield high rates of primary production and that can tolerate such high levels of grazing.  Further, intense grazing and frequent nutrient inputs from herbivores yields rapid nutrient cycling. Although terrestrial ecosystems, such as the savanna grasslands of the Serengeti, Yellowstone National Park, and Konza Prarie are typically thought of as archetypal grazing ecosystems, many marine ecosystems have similar characteristics. Here, I focus on the comparisons between coral reefs and savanna grasslands – two grazing ecosystems that have remarkable parallels despite their obvious differences. Much like savannas, coral reefs have roving herds (i.e. schools) of fishes that graze the ecosystem intensely, regulate primary production, determine primary producer standing crop, and facilitate nutrient cycling. Also similar to savannas, coral reefs have different functional groups of herbivores (grazers vs. browsers) that play different but important roles in ecosystem function, and the idiosyncratic removal of these species results in degradation of ecosystem function. Herbivory is not the only important trophic interaction in these ecosystems:  top predators (lions vs. sharks) are also now recognized as important determinants both of herbivore population dynamics and of the resulting impact on plant communities. One remarkable characteristic of both wet and dry ecosystems is that both often depend on herbivory to maintain ecosystem integrity, and the loss of herbivores due to overharvesting, hunting, or disease, results in a fundamental reorganization of these ecosystems, with coral reefs becoming dominated by large macroalgae instead of corals and savannas becoming dominated by woody vegetation instead of grasses.Ultimately, comparing the ecology of marine vs. terrestrial grazing ecosystems will help us develop broader theories of herbivore ecology and of how herbivores structure and maintain ecosystem function.