I bought this mainly for running in rain. It's very thin and does not have any kind of liner on inside so you won't get hot. It will not keep you warm. It does help block wind. I've worn it running twice in the rain. The first was a downpour for about 5 minutes and then about an hour of light rain. Interestingly the hood worked very well at keeping my hair dry once I figured out how to tie the pull strings together under my chin, but my torso and arms were pretty wet. Not as much as I would have been without the coat. The second time it was light rain for about an hour and my shirt was damp all over when I took off the coat. The water seems to stay out for a little while but I feel like it starts soaking through once it reaches its saturation point. It kept me dry enough to keep running (no fun running in soaked clothes) but the jacket is definitely not waterproof, just water resistant. The medium fit well and I am 5'7" 160lb, 37" chest.