The aim of this report is to present work on threee processing methods with the potential to improve the performance of active sonar systems. Advantages of the processing methods are discussed and illustrated with simulation results. In addition, simulation packages are presented. Further work needed for a more detailed evaluation of the methods is indicated. We describe the development of software for an active sonar processor for a monostatic transceiver and a bistatic receiver including Doppler processing. Combined mono- and bistatic processing yields advantages in terms of covert operation of the bistatic receiver. Improved detection probability can be achieved and increased resilience to countermeasures. The objective has been to obtain a reference for comparison with new advanced sonar processors and a tool for accurate performance predictions. In addition, initial results from two promising methods are included. Synthetic Aperture Sonar (SAS) processing, commonly used in advanced mine hunting sonars, is here utilized at lower frequencies for surveillance applications. Space-Time Adaptive Processing (STAP), a signal processing technique used in radars to enhance the ability to detect targets that might otherwise be obscured by clutter or by jamming, can be applied in active sonar in order to reject reverberation. SAS has potential for improving the detection and classification performance of conventional sonars. Simulations made under idealised conditions indicate that good results can be obtained for relevant distances and frequencies. Fully adaptive STAP processing requires large amounts of training data, which is not always readily available in sonar applications. We present some alternative approaches that require less training data, with promising results.