I originally bought the large, but returned for the x-large. My biceps are larger than normal and I don't have a 'runners physique' ... the jacket has good pockets, nice finger holes to keep your hands a tiny bit warmer without gloves. I'm in VT and run hot (thank you perimenopause!) so enjoy staying cool ... this jacket gives me a bit of extra warmth in the 40 degree weather, but still breathes. Wish the arms were a touch wider so I could fit a thick hoodie underneath for winter...but for now, it works great. Color is perfect for being seen.