HVDC has been a viable technique since the 60's and has played a vital part in both long distance transmission and in the interconnection of systems. Its relative economic position has been much discussed over 30 years and this report is both an update of those considerations and a review of the present position. It reviews many aspects of transmission development common to both AC and DC transmission with a concentration on the specific aspects of DC as they give advantage or extra dimension to the long distance transmission of electrical power.