To make possible for the defense to transfer data under water, between vehicles, FOI has developed a number of communication methods to be used in different situations. In this report mainly the activities during 2011-2014 are described, but to give the full picture a number of experiments done outside of this time period and also in other projects are included. The report starts with an outlook, compiled from open sources, of the present state in other naval forces concerning the use of underwater communication. As shown, the development has gone far in some places and the technology is in operational use. The physical layer consists of the link between two communication nodes. Over the years a substantial part of the work has been spent in order to be able to cope with the difficult properties of the acoustic underwater channel. Several methods have been developed with different data rates and ranges. Results from experiments with these methods and their properties are presented. A communication system does not only consist of two nodes in each end of a transmission channel. When several nodes are connected together, a network protocol is needed to establish communication, free of interference, between different platforms. In section 4 such a protocol, developed especially for underwater communication, is shortly described. A modem to transmit digital speech, short text messages (SMS) and arbitrary files, for example images, has been developed by FOI. A short introduction to this modem in particular and a short discussion about some aspects around modem development in general are given. The project underwater communication has been a partner in several international cooperations, e.g. RACUN which recently was finished with a large experiment in the Mediterranean sea. New international cooperations are under planning and these are shortly described in section 7.3. Finally, the outcome of the project in terms of communication methods, experiment equipment, and increased knowledge is presented. In the report recommendations are given for how the research results should be taken care of, to benefit the navy activities, and in which areas new research should be done. In the Appendix it is given, for those who wish to dig deeper, more detailed descriptions of the basics for the FOI communication methods and the latest experiment in Djupviken 2014.