Depends on your tolerance for cold weather. I was planning a camping trip in October a couple of years ago and ended up not doing it because while I could have

Depends on your tolerance for cold weather. I was planning a camping trip in October a couple of years ago and ended up not doing it because while I could have managed, I wasn't really prepared to deal with sub-freezing high temperatures and it ultimately didn't sound like much fun anyway. On the flip side, I've heard that Yellowstone is one of those places that is so popular that it's overrun with tourists to the point of no longer being fun during the peak season. I think the best time to go is therefore either very early in the spring or very late in the fall when it's cool, but not miserably cold.