We discuss in general terms, factors which determine the luminescence efficiency of various luminescent materials and devices. These processes can be classified into two broad categories: the first has been thoroughly studied and entails interactions which lead to non-radiative dissipation of optical energy at the specific active center. The second concerns interactions leading to the delocalization and the transport of optical energy away from the originally excited state. Recent spectroscopic results from various studies have allowed the absolute placement of the ground state of luminescent ion series within the band gap of the host crystal and have cast light on the systematics of such placement. These results are relevant to the ultimate efficiency of phosphors systems as they determine whether the excitation is localized.