I was there the first week of August last year and wore shorts and t-shirt every day.

Crowds?

The only time it really felt crowded was the Jenny Lake hike on a Sunday. Obviously, if I had it to do over I would do that hike on Monday.

Also, the parking lot for the Mt Washburn hike filled up but the trail was very spread out.

Even the Old faithful deck was reasonable. A few crowds in some places such as Grand Geyser. But nothing bad.

I would do that again.

Okay. I have an opinion now. I just looked at a friend's pictures of the Tetons in the fall. Big difference from my pics in June. I'd definitely go earlier for more snow on the peaks.

But, Starrs, then you'll miss the beautiful foliage!

Oh, darn. :-(
Let me think about this....
:-)

No matter when you go it's going to be a great experience!!!!

Utahtea

My vote: September. Aspens and tamaracks will be changing colors. And after growing up and living in the area most of my life, in general, you'll find better weather in September than June (especially early June). You'll have cool mornings but you're less likely to have rain. However, we ran into snow in Glacier over Labor Day. ;)

But basically, overall, with all the weather caveats, etc.: better weather in September than June. September is my favorite month in MT.

We were there the last week in August 10 years ago and I thought it was the perfect time to go. We were in shorts during the day and the nights were comfortably cooler. While there were lots of other visitors I didn't find it to be crowded.