Technical Abstract: Plants are eaten by a tremendous array of herbivores, including almost half of known insect species. Although herbivores consume most plants to some degree, many plants are also relatively well defended, and herbivore populations are often limited by predators. The net result of these processes is that the extent of herbivory, and the strength of its effects on plant populations and ecosystem processes, are highly variable. Herbivores strongly influence plants in aquatic environments, productive environments, and environments with nutritious, poorly-defended plants or few predators. Effects of herbivores can also be accentuated through positive feedbacks between herbivory and soil resources, in which herbivore-mediated changes in plant community composition accelerate or decelerate nutrient cycling. Although research on herbivory has focused primarily on natural systems, few of these systems are free of human influence. Humans dramatically change plant-herbivore relationships by removing predators, increasing plant resource availability, and moving both plants and herbivores around the globe.