The "Where?" is quite important for Mixed Reality applications: Where is the user looking at? Where should augmentations be displayed? The location of the overt visual attention of the user can be used both to disambiguate referent objects and to inform an intelligent view management of the user interface. While the vertical and horizontal orientation of attention is quite commonly used, e.g. derived from the orientation of the head, only knowledge about the distance allows for an intrinsic measurement of the location of the attention. This contribution reviews our latest results on detecting the location of attention in 3D space using binocular eye tracking.