Power system planning and operation of a vertically organized electric utility used to be relatively simple because the decision makers are inside one organization. The regulatory environment also used to be simple, with few external organizations having jurisdiction over the electric utility. It was often sufficient for a utility to decide what type of generation would be built to meet future demands, and where and when it would be built. Transmission lines were planned and built to move the generated power to the consumers with few constraints. This changed when the electric utilities were divided into separate generation, transmission, and distribution entities. Furthermore, approvals for any system expansion became much more complex with many more political entities involved in the approval process. The power system planners are faced with having to evaluate numerous technology options for any expansion or change of the existing power transmission systems. Power electronic systems for both alternating current (AC) and high voltage direct current (HVDC) systems have evolved to the point that it is not obvious if AC or HVDC is the best economic option for satisfying the power transmission needs. Technology turn-over is also accelerating. One driver for the new technology developments is the requirement to build more renewable electric power generation systems, which the existing power transmission systems cannot accommodate. This chapter aims to provide technical information about AC and HVDC technology options to enable the planners to make informed decisions about which technology option that might be preferred for the required power system expansion or change.