Don't wear a Helmet. Can't stand them. Would not wear it under a Helmet though. The Hood is Huge and infinitely adjunstable, I would just open it up and wear it over the Helmet like I do with the Cap I ride with. Doubt if it would fit under a Helmet. This Fabric is really thick. It is something called 3 layer nylon Professional Grade. It is their top of the line severe weather jacket. The Pant too.