This "Brainsaver" model from Triple 8 features a 360 degree wrap around design to prevent liner seperation, a moisture wicking layers limiting sweat drippage, and an anti-bacterial treatment which covers most odors. Each helmet comes CPSC certified. Sizing: Small = head circumference of 20.5in. - 22.5in. Medium = head circumference of 21.5in. - 23.5in. Large = head circumference of 22.5in. - 24.5in. Large = head circumference of 23.5in. - 25.5in.