http://www.nps.gov/yell/index.htm
The world’s first national park is also one of it’s most famous camping destinations. Play the licence plate game, and you’ll collect cars from almost every US state and Canadian province. They have all come to enjoy Yellowstone’s natural beauty and abundant wildlife.
Yellowstone’s 12 campgrounds vary in terms of whether they can be booked in advance , whether they have hook-ups, flush toilets, and even whether you can stay there in a canvas-sided pop-up (due to all the bears.) There are also plenty of private campgrounds just outside the park boundaries. Be sure to research before you go.
Also research how cold Yellowstone gets; our mid-summer night-time temperatures were close to freezing. Bring blankets.