In this chapter, we introduced you to the concept of 3D rendering, the 3D virtual camera, and the notion of 3D overlay for Augmented Reality. We presented what a virtual camera is and its characteristics and described the importance of intrinsic camera parameters for accurate Augmented Reality. You also got a chance to develop your first 3D overlay and calibrate your mobile camera for improved realism. However, as you move your phone along, the video background changes, while the 3D models stay in place. In the next chapter, we will tackle one of the fundamental bricks of an Augmented Reality application: the registration.