AC electric power transmission systems are highly interconnected because they provide the best access to economic power sources, enable the sharing of spinning reserves, and reduce the consequences of loss of generators and lines. That is, it reduces the negative impact caused by equipment and plant outages although it also introduces possibilities for events causing large and costly outages. The trend for bulk power transfers and interconnection of remote power sources is to use long underground or submarine cables, which might only be possible if HVDC technologies are used. To provide similar power supply security as for AC systems, HVDC system interconnections are technically feasible by using Voltage Sourced Converter (VSC) feeding cables or overhead lines. This requires HVDC circuit breakers or switches capable of forcing fault currents to zero. Such breakers have been demonstrated, which means that for the most part the components needed to construct HVDC grids are available. A demonstration system of a meshed DC grid has even been built in China. This chapter describes the technologies and developments of HVDC grid technologies but also identifies some barriers, for example the costs of converters and breakers still to be overcome.