All helmets attempt to protect the user's head by absorbing mechanical energy and protecting against penetration. Their structure and protective capacity are altered in high-energy impacts. Beside their energy-absorption capability, their volume and weight are also important issues, since higher volume and weight increase the injury risk for the user's head and neck. Anatomical helmets adapted to the inner head structure were invented by neurosurgeons at the end of the 20th century

Helmets used for different purposes have different designs. For example, a bicycle helmet must protect against blunt impact forces from the wearer's head striking the road. A helmet designed for rock climbing must protect against heavy impact, and against objects such as small rocks and climbing equipment falling from above. Practical concerns also dictate helmet design: a bicycling helmet should be aerodynamic in shape and well ventilated, while a rock climbing helmet must be lightweight and small so that it does not interfere with climbing.

Historically, helmets have been made from a wide range of materials, including various metals, plastics, leather, and even some fibrous materials such as Kevlar. Ancient and medieval helmets were usually made of metals, often bronze, iron or steel

Helmets of many different types have developed over the course of human history. Most early helmets had military uses, though some may have had more ceremonial than combat-related purposes.
Helmets of