Some warm days, nothing under the helmet. Other warm days, a wicking beanie, particularly if I don't want my super-short hair sticking up through the vents. This is usually the case on commute days- it's hard to get rid of the wookie hair at work. Hmmm...maybe I should start rocking the wookie at the office? A little hair gel...

Cold days, either a thin wool beanie or, if it's really cold, a balaclava.