Plants in the Arctic and subarctic face the problems posed by herbivory in addition to short growth seasons, low temperatures and low nutrient availability. Herbivores control plant performance by removing biomass, by altering resource availability, by altering the physical environment, and by changing the balance of competition. The main difference between effects of herbivores in the Arctic and at lower latitudes may be the relatively greater importance of changes in resource availability and the physical environment resulting from herbivore activity, and their consequences for plant competitive abilities. Species responses to defoliation depend primarily on growth form. Artificial defoliation of graminoids has negative effects on most species, but in the field total effects of herbivores are often neutral or even positive, resulting in increased nitrogen concentrations in shoots in many species. Shrubs are less able to respond positively to herbivory than graminoids, and although there is some evidence that deciduous shrubs recover faster than evergreen ones, the difference is not great. However, effects of herbivores on shrubs are little studied, despite their importance in the herbivore diet. Responses of individual species to increased nutrient availability vary greatly, even within a growth form. Some graminoids and shrubs show strong positive responses to fertilization while others show little or no response. These species-specific effects suggest that herbivores can alter interspecific relationships through differential responses to fertilization. Herbivores may alter plant population dynamics by altering flower or seed production, by consuming seedlings, or by altering the availability of microsites. However, no study has adequately examined this for any arctic species. Changes in community composition following removal of herbivores are the result not only of selective removal of some plant species, but also of changes in microsite availability, nutrient availability, litter accumulation, and soil characteristics. Thus, the view that abiotic factors are the overwhelming determinants of community structure in low-productivity environments is compatible with the view that herbivores exercise their influence to a large extent by altering abiotic factors. Arctic herbivores often increase total above-ground nitrogen availability (and therefore food quality) in the plant community, but increased productivity as a result of herbivores is rare. The increase in nutrient availability is probably due in part to changes in soil temperature and soil moisture following a reduction in litter accumulation. Although our knowledge of effects of herbivory on individual plants and on communities is extensive, we lack information on effects at the population level. We also do not have an adequate understanding of impacts of herbivores at different spatial and temporal scales, something which is needed to be able to make predictions about longer-term impact of herbivores in these systems.