Medium = 54-58cm. Needs new batteries, requires (3) LR44. To change batteries: Wiggle the white foam liner out, or at least as far as you need to expose the brown plate behind the on/off switch. Lift that panel out, and there are three watch batteries in there (LR44 I think???). Be gentle, as there are thin wires carrying power to the led, and they are routed and taped along the foam liner and the helmet cover. Not hard to do, but just keep it in mind.
