Introduction. Mechanisms and consequences of overvoltages. Atmospherical (external) and switching (internal) overvoltages. Lightning. Lightning strikes on buildings, technical installations, transmission and distribution electric networks. Static electricity: formation, dangers and protection. Hazards arising from a lightning strike. Protection of human beings, buildings, technical installations, transmission and distribution electric networks against lightning and overvoltages in general. Effects of the electric field and current on human beings. Electrocution: mechanism and protection.