VirtuaHead is a practical and robust head-position tracking method using computer vision. By combining two simple image processing techniques, this tracker can report the position of the user's head in real time. Whole image processing is performed by software running on normal mid-range workstations. This tracker can support desk top virtual reality (also referred to as ``fish tank VR''), thereby enabling a user to use a wide range of 3D systems without having to put on any equipment. An experiment conducted by the author suggests this tracker can improve the human's ability in understanding complex 3D structures presented on the display.