For protection against divers and other small underwater threats in harbours or temporary established forward bases, active sonar systems offer the most developed counter technology. Such sonar systems are available from a great number of manufacturers and are used for harbour surveillance to an increasing extent. At present, there is no system in operation in Sweden, still the ability to improve the capacity of underwater surveillance in harbour areas are discussed in military as well as civilian applications. Different properties and variations in the underwater environment are the limiting factors for active sonar performance in harbour environment. In the project "Under-water sensor systems for harbour and base protection" data fusion with input from passive underwater systems, (sonar, electric and magnetic) is studied. The goal of this project is to support active sonar with passive systems, so a more complete coverage and protection can bee provided. The use of active sonar during realistic conditions in the Gothenburg harbour has shown both the potential and some of the limitations with active systems. Examples from literature point to the active sonar capabilities as well as the limitations in performance due to the complicated underwater environment usually found in a harbour area.