Of Lamar Valley's abundant wildlife, the most famous – and to some, controversial – resident is the gray wolf. Wolves were missing from the ecosystem between 1926, when the last pack was killed, and 1995, when 31 Canadian wolves were re-introduced to the park. The population has stabilized at around 100, but continual pressure from lobbying groups has seen the species bounced on and off protected lists. Outside the park, it is currently legal to hunt wolves in Montana, Idaho, and most of Wyoming.

　　In the northwest area of Wyoming around Yellowstone, wolves can only be hunted from October 1 to December 31. It is illegal to hunt wolves in the park itself.

　　Re-introducing a top predator to the ecosystem provides an unprecedented look at how species are interconnected, and scientists have been watching closely. As wolf numbers have increased, coyotes have decreased (wolves kill them but don't eat them), meaning small mammals and rodents have rebounded – good for birds of prey. Grizzlies consistently steal fresh kills from wolves, which provides them with another food source during lean times. Some attribute an increase in willow growth to a decrease in elk in the wake of the wolves' re-introduction. Visitor numbers have increased in this remote corner of the park as people stare intently into the hills hoping to catch a glimpse of these magnificent animals.