The latest generation of consumer market head-mounted displays (HMD) now include self-contained inside-out tracking of head motions, which makes them suitable for mobile applications. However, 3D tracking of input devices is either not included at all or requires to keep the device in sight, so that it can be observed from a sensor mounted on the HMD. Both approaches make natural interactions cumbersome in mobile applications. TrackCap, a novel approach for 3D tracking of input devices, turns a conventional smart-phone into a precise 6DOF input device for an HMD user. The device can be conveniently operated both inside and outside the HMD’s field of view, while it provides additional 2D input and output capabilities.