This article reviews dc transmission technologies for future power grids. The article emphasises the attributes that each technology offers in terms of enhance controllability and stability, resiliency to ac and dc faults, and encourages increased exploitations of renewable energy resources (RERs) for electricity generation. Discussions of ac/dc and dc/dc converters reveal that the self-commutated dc transmission technologies are critical for better utilisation of large RERs which tend to be dispersed over wide geographical areas, and offer much needed controllability for operation of centralised and decentralised power grids. It is concluded that the series power flow controllers have potential to restrict the expensive isolated dc/dc converters to few applications, in which the prevention of dc fault propagation is paramount. Cheaper non-isolated dc/dc converters offer dc voltage tapping and matching and power regulation but they are unable to prevent pole-shifting during pole-to-ground dc fault. To date hybrid dc circuit breakers target dc fault isolation times ranging from 3 to 5 ms; while the resonance-based dc circuit breakers with forced current zeros target dc fault clearance times from 8 to 12.5 ms.