This is the final report of the project "Skydd av marint liv vid användning av aktiv sonar", which was carried out by FOI, the Swedish Defence Research Agency, during 2012 and 2013. Commissioned by FMV, the Swedish Defence Material Administration, this project has strived to estimate the environmental impact of the Swedish Navy's use of active sonar and find ways of minimising this impact. The project has participated in the European Defence Agency project Protection of Marine Mammals (PoMM), where scientists and decision makers from Germany, Italy, the Netherlands, Norway, Sweden and the United Kingdom have developed knowledge and methods for protection of marine mammals during active sonar exercises. The impact of active sonars on marine life has come into focus in recent years due to a number of mass strandings of whales and dolphins. These mass strandings have been connected to naval use of active sonar. In response to this, several countries have implemented research programs aimed at estimating and minimising the environmental impact of active sonars. These programs have focused on effects on marine mammals because they have displayed the greatest impact and because their hearing is better than that of fish at typical sonar frequencies. In Sweden, research into the environmental impact of naval active sonars has not previously been conducted. The activities and capabilities of the Swedish Armed Forces are considered of national importance. Active sonar is crucial for detection and classification of objects in the water such as mines and submerged submarines. It is therefore important that the Swedish Navy maintain their active sonar capabilities. In this report we show that with knowledge, appropriate tools, and simple guidelines, the Swedish Navy can modify its doctrine so that active sonars are used in an environmentally responsible fashion while the effects on capabilities and performance are negligible. We recommend that the Swedish Navy proceeds to implement these changes. This report introduces the project's activities and summarises its results; full details are available in reports, memos and scientific publications listed at the end of the report. Within PoMM we have contributed to the development of a common European database on marine mammals, a graphical interface for audio analysis, and methods of detection and classification of marine mammal calls. We have summarised information about marine mammals in Swedish waters, i.e. harbour porpoises, grey seals, harbour seals, and ringed seals, and assessed the legal situation with regards to the protection status of these animals and the use of active sonar in Swedish waters inhabited by marine mammals. We have conducted experiments to estimate the impact of Swedish Navy sonar pulses on porpoises and seals and showed that harbour porpoises may receive severe behavioural effects at ranges up to 20 km from a transmitting sonar. We have evaluated software tools that support the Navy in managing risks of disturbing or injuring marine mammals during active sonar exercises and recommend that such a tool is used. Finally, we have proposed guidelines for environmentally responsible use of active sonar in the Swedish Navy.