The term seabed warfare, though insufficiently defined, seems to be increasingly important. Unmanned, and specifically autonomous, underwater vehiclesare generally expected to play an essential role in seabed warfare.In addition to smaller unmanned vehicles, which have been used for many years,the focus has lately shifted to what is usually called XLUUVs – extra large unmanned underwater vehicles, with a diameter of more than two metres. These large unmanned submarines could, through their long range and endurance, be suitable systems for seabed warfare outside ports and other confined areas. Some of the largerunmanned submarines can be launched from submarines and thus offer a uniquecapability for covert insertion of the vehicles.The development and employment of XLUUVs is accelerating worldwide, withsystems coming into service in several states, some developed by commercial companies and others on order by armed forces. Two suitable roles for unmanned submarines in a Swedish context could be as patrol vehicles and as decoys to protectmanned submarines