PREFACEThis Special Issue presents selected papers from EERA DeepWind’2018, 17 – 19 January, Trondheim, Norway. This was the 15th Deep Sea Offshore Wind R&D Conference. The conference is hosted by SINTEF and NTNU and organized in cooperation with the European Energy Research Alliance (EERA) joint programme on wind energy. A total of 37 papers are included addressing the research topics of the conference:▪ New turbine and generator technology▪ Grid connection and power system integration▪ Met-ocean condition▪ Operation and maintenance▪ Installation and sub-structures▪ Wind farm optimization▪ Experimental testing and validation▪ Wind farm control systemsAll papers presented in this special issue have gone through a careful peer-review organized by the international Scientific Committee of EERA DeepWind’2018:Anaya-Lara, Olimpo, Strathclyde UniversityBredmose, Henrik, DTUBusmann, Hans-Gerd, Fraunhofer IWESEecen, Peter, ECNFurevik Birgitte R., METHeggelund, Yngve, CMRJørgensen, Hans Ejsing, DTUKvamsdal, Trond, NTNULeithead, William, Strathclyde UniversityMadsen, Peter Hauge, DTUMerz, Karl, SINTEF EnergiMuskulus, Michael, NTNUNielsen, Finn Gunnar, UiBNygaard, Tor Anders, IFEReuder, Joachim, UiBRobertson, Amy, NRELRohrig, Kurt, Fraunhofer IEETande, John Olav, SINTEF EnergiUhlen, Kjetil, NTNUVan Bussel, Gerard, TU DelftVan Wingerde, Arno, Fraunhofer IWESWelte, Thomas, SINTEF EnergiWiggert, Marcel, Fraunhofer IWESØkland, Ole David, SINTEF OceanAcknowledgementsThe financial support from the Research Council of Norway covering a share of the cost of organizing the conference is greatly appreciated. The conference chairs and guest editors of this Special Issue like to express our sincere gratitude to the Scientific Committee of EERA DeepWind‘2018, all authors that have contributed to this Special Issue, and to colleagues that have so kindly contributed to the review process. We hope that this Special Issue provides results that contribute to the fruitful development of offshore wind energy. Details of the DeepWind’2018 conference and all conference presentations and posters are available at the conference web page.Conference chairs and guest editors:John Olav Giæver Tande, Chief Scientist, SINTEF Energi Trond Kvamsdal, Professor, NTNU Michael Muskulus, Professor, NTNU