The BoneShields H31G Helmet is made for protection during skating, bicycling, skateboarding, and other activities. It features an “adjustment fit” system at the back of the helmet which allows the size to be dialed up or down for a more precise fit. Size Small fits head circumference 48-54cm. Size Medium fits 55-58cm. Size large fits 59-61cm. This helmet complies with CPSC Safety standards for ages 5 and older.