This CIGRE Green Book on High Voltage Direct Current (HVDC) Transmission Systems is intended to assist electrical engineers and power system planners and others to develop an understanding of how to select, apply, and manage power electronic systems for conversion of AC to DC and vice versa and how to integrate HVDC links in AC systems. This introductory chapter provides some background as to how HVDC power transmission systems function and gives a historical perspective of how they have evolved to become an indispensable component of power systems infrastructure all over the world as an alternative to AC circuits for long distance transmission of electric power via overhead lines and cables.