As described in the preceding chapters, nitrides and oxynitrides activated with rare earth ions have, in many cases, luminescence properties suited well to white LED application. Particularly important are strong blue or near-UV absorption resulting in ef”cient visible luminescence and small thermal quenching of luminescence. It is still not clear enough, however, why these features are brought about, although research has been done to deepen understanding of such fundamental properties in parallel with the search for new materials. In this chapter, the features of nitride and oxy nitride phosphors, in comparison with conventional phosphors, are revisited, with an expectation that such a comparison can make clear the physical properties of these new phosphors.