The short answer
Yellowstone requires visitors to stay at least 100 yards from bears and wolves and 25 yards from all other wildlife, including bison and elk. Carry bear spray on every hike, use pullouts rather than stopping in the road, and slow down at dawn and dusk.
Distances that matter
Bison injure more Yellowstone visitors than any other animal. Use binoculars and zoom lenses instead of walking closer, and if an animal approaches you, back away.
Bear spray
Carry it on every hike — accessible on a hip or chest holster, not in a pack. You can’t fly with it, so buy it in Bozeman or a gateway town.
Wildlife jams
When cars stop for animals, pull completely into a pullout if one is available, stay in the van near large animals, and keep moving if there’s no safe place to stop.
Driving at dawn and dusk
Deer, elk and bison cross highways like US-191 and US-89 most often in low light. Slow down, use high beams when safe and scan the shoulders.