These run large. I wear a large and bought that size and it was too loose. I violated washing instructions by washing it in "warm" and throwing it into the dryer. It now fits perfectly.

Before risking excessive shrinkage by washing it and drying it, I bought a medium, and that will be too tight untiL I get out of  my long hair phase or my head shrinks as I age.

Like the others say: comfortable and the vents are great.