The fusion of ultrasound data with a tomographic modality like CT or MRT is beneficial for many applications, e.g. intra-operative navigation scenarios. In particular it can also support the treatment planning and delineation of the target volume for radiation therapy. We investigate methods for image-based registration and fusion of ultrasound images with CT/MRT scans. This includes techniques for tracking, calibration and compounding for 3D freehand ultrasound, registration algorithms, as well as visualization and navigation methods.