Supposedly, this jacket is made of a more breathable material than anything else that's really water proof. It's certainly better than anything else I own (Gore-tex, Karbonite.) But the jacket also has ginormous armpit zips! So if it's not breathable enough for aerobic activity, and in my climate, to be honest, that's pretty frequent, the zips help a ton. I'll even wear it on a crisp dry day instead of my wind breaker. Partly because of the zips, and partly because hey, it could rain.