In order to present a simulated environment that one can navigate through and interact with, it is necessary to know the exact position of the person in the TrackingLab. For this, a specially designed Vicon Co. camera system continuously records every move- ment of the person by means of reflective markers that are attached to the body. This position data can then be transmitted to a computer which the persons carry with them in a rucksack. The computer then uses the data to generate the virtual environment (consisting of 3D images and 3D sounds) in real time. These images and sounds correspond at every moment to the person's actual position in the room and are experienced through so-called Head Mounted Displays (or HMDs for short), which are specially designed spectacles with tiny screen displays.