This chapter discusses the design and testing of semiconductor valves for HVDC power transmission. Starting with a historical introduction to the development of HVDC and of HVDC mercury arc valves, it explains why semiconductor-based valves became established in the 1970s. Two fundamental types of valves have been used in HVDC schemes: Line Commutated Converter (LCC) valves which use thyristors, and since the late 1990s, Voltage-Sourced Converter (VSC) valves which use IGBTs. The latter category can be further subdivided into valves that function as controllable switches and valves that function as controllable voltage sources. There are significant differences in the design characteristics between these three types of valves, although certain aspects are common to all. These include the need for high-voltage external insulation, high-efficiency forced cooling and a means for transmission of command information from ground potential across a high insulating barrier. The major components and design challenges for each type of valve are discussed, along with a short discussion on the electrical testing usually performed on such valves.