Human exploration of the world oceans is ever increasing as conventional industries grow and new industries emerge. A new emerging and fast-growing industry is the marine renewable energy. The last decades have been characterized by an accentuated development rate of technologies that can convert the energy contained in stream flows, waves, wind and tides. This growth benefits from the fact that society has become notably aware of the well-being of the environment we all live in. This brings a human desire to implement technologies which cope better with the natural environment. Yet, this environmental awareness may also pose difficulties in approving new renewable energy projects such as offshore wind, wave and tidal energy farms. Lessons that have been learned is that lack of consistent environmental data can become an impasse when consenting permits for testing and deployments marine renewable energy technologies. An example is the European Union in which a majority of the member states requires rigorous environmental monitoring programs to be in place when marine renewable energy technologies are commissioned and decommissioned. To satisfy such high demands and to simultaneously boost the marine renewable sector, long-term environmental monitoring framework that gathers multi-variable data are needed to keep providing data to technology developers, operators as well as to the general public. Technologies based on active acoustics might be the most advanced tools to monitor the subsea environment around marine manmade structures especially in murky and deep waters where divining and conventional technologies are both costly and risky. The main objective of this PhD project is to develop and test active acoustic monitoring system for offshore renewable energy farms. This was done by integrating a multitude of appropriate monitoring sonar, hydrophones and cameras systems to be developed with standards suitable for subsea environmental monitoring. A first task was to identify, then secondly, acquire and test sonar systems. A platform was designed and built, a data acquisition device control systems were developed, and finally, additional instruments such as video cameras and sonars were added. Then a systems integration was followed by calibration of devices. The sonar systems were used for quantitative measurements of the occurrence of e.g. schools of fish near marine renewable energy converters. The sonar systems were also used for seabed inspections, depth measurements and cavitating flow observations. So far, the combination of multibeam and dual-beam sonar systems produced good results of target detection, bottom inspection, depth measurements and biomass estimation. The multibeam sonar system was capable of resolving isolated targets located near high acoustic retroreflective objects. Panoramic acoustic images of wave and instream energy converters were acquired using a multibeam sonar operating at frequencies near 1 GHz. The Dual-beam and split-beam sonar systems produced data referent to acoustic background intensity of targets that helps classifying targets according to its size, composition and 3-Dimensional location within the water column. During the next phase of this project, the platform will be deployed for longer periods in order to gather consistent acoustic and optical backscattering data of e.g. marine animal occurrence within marine renewable energy farms.