To transmit electric bulk power in an economic and efficient way over long distances up to several 1000 km it is necessary to use Ultra-High-Voltages (UHV), i.e. AC (alternating current)-voltages of 1000 kV and DC (direct current)-voltages of +/- 800 kV or higher. Especially the UHVDC (ultra high voltage direct current) transmission is one of the key solutions to transmit bulk power over very large distances. Two bipolar systems with transmission voltage of +/- 800 kV, transmission power up to 7.2 GW and length of up to 2000 km are in operation in China since 2010 and additional systems especially in China and India are planned. So it seems that the UHVDC technology is state of the art and can be used easily. But there are some restrictions which make it difficult to use UHVDC transmission in all cases. To analyze this in detail, the state of the art of long distance UHVDC transmission systems is summarized and the possible future development is analyzed. Different aspects are discussed, which will possibly limit the use of the UHVDC technology in future. (C) 2012 Elsevier Ltd. All rights reserved.