A modular helmet system having a helmet base to protect from impacts the crown of a wearer's head and terminating basally in a rear rim and side rims; a C-shaped extension detachably connected to said rims to protect from impacts, when fitted to the helmet base, the base of a wearer's skull; and a crown comb detachably connected centrally over the crown of the helmet base. The helmet base is provided with ventilation windows centrally located over its crown. The comb covers and seals said ventilation windows. The comb is also employed to secure the extension in place, when fitted on the helmet base.