The reintroduction of gray wolves (Canis lupus) to Yellowstone National Park is the most celebrated ecological experiment in history. As predicted by population models, the rapid recovery of a wolf population caused both temporal and spatial variability in wolf–ungulate interactions that likewise generated temporal and spatial variation in the expression of trophic cascades. This has amplified spatial variation in vegetation in Yellowstone, particularly with willow (Salix spp.) and cottonwood (Populus spp.) in riparian areas, with associated changes in food webs. Increasing influences of grizzly bears (Ursus arctos), cougars (Puma concolor), and bison (Bisonbison) are making what initially was predominantly an elk–wolf interaction into an increasingly complex system. Outside Yellowstone, however, humans have a dominant influence in western North America that overwhelms trophic cascades resulting in what appear to be bottom-up influences on community structure and function. Complex and unexpected ecosystem responses to wolf recovery in Yellowstone reinforce the value of national parks and other protected areas as ecological baseline reserves.