1. A device for projecting patient image data from radioscopic imaging methods and/or tomographic imaging methods onto video images, comprising an image display device ( 10 ), at least one video camera ( 14 ) and a computer-assisted navigation system which can detect the spatial positions of said image display device ( 10 ) and/or said video camera ( 14 ) and the spatial positions of a part of a patient's body ( 1 ) via tracking means ( 12, 3 ) attached to it, characterized in that said image display device ( 10 ) and said video camera ( 14 ) are assigned to each other and are realized as a portable unit.