Surge-protective devices (SPDs) used in transmission and distribution overhead lines (OHLs), while the second is related to low-voltage (LV) distribution systems. Metal oxide arresters (MOAs), with and without external gaps, and multi chamber arresters (MCAs) are used for protecting high-voltage (HV) and medium voltage (MV) lines against lightning overvoltages. An SPD is a device that is intended to limit transient overvoltages and divert currents. A voltage limiting type SPD has high impedance when no surge is present, but this impedance is continuously reduced with increased surge current and voltage. The main causes for surges are direct or indirect lightning strikes and man-made switching overvoltages. For an efficient protection of LV power systems against voltage or current surges, so-called LV surge protective devices are frequently used. The second part of this chapter gives an overview about surge protection for LV power systems.