They are made with a hard, plastic shell on the outside with a thick padding lining on the inside. When you slam against hard asphalt, the materials absorb the energy of the impact and dissipate the force, reducing the pressure from your skull. Different skateboarding disciplines require different forms of helmets to be used.  There are usually two types of helmets: half-shell and full-face helmets. While the former is less expensive and is generally used for cruising, freestyle, street skateboarding, and similar activities, the latter is more protective and is recommended for downhill skateboarding.