White-tailed deer have increased in abundance and expanded their geographic range in North America over the past century, and now exist at higher densities than they have in the past several hundred years. This is having numerous impacts on the forest ecosystems they inhabit. Regional recruitment failure of eastern hemlock (Tsuga canadensis) and northern white cedar (Thuja occidentalis) trees can be explained in part by deer browsing. Deer also have significant negative effects on understorey plants, including wild lily-of-the-valley (Maianthemum canadense) and white-flowered trillium (Trillium grandiflorum). Long-term studies of primary, old-growth forest stands reveal a 48–81 per cent herb and shrub species loss accompanying increases in deer density. Graminoids, ferns and club mosses were more likely to persist in these stands than plants in all other taxonomic groups. Deer also exhibit indirect effects on forest communities by reducing host plant densities or altering forest structure. Because of their numerous direct and indirect effects on other species, and because of the magnitude of these effects, white-tailed deer act as a keystone herbivore. Natural regulation and maximum sustained yield management approaches have failed to alleviate deer impacts on forest ecosystems, but an ecosystem-based management approach offers promise.