Phosphor has a history of more than 100 years; now it is almost everywhere in our daily life. For example, we turn on –uorescent lamps, switch on the computer in our of”ce, turn on the TV to watch programs, and even see x-ray images of our chest, brain, or legs at the hospital. The light or the colorful images we see are generated by means of some phosphors, such as lamp, cathode-ray tube (CRT), or x-ray phosphors. These phosphors have been extensively studied, and notable progress has been made in their ef”ciency and quality over the past 40 years. On the other hand, as a de”nite type of phosphor, LED phosphors are quite new to the family of luminescent materials, which appeared after white LEDs came onto the stage several years ago. As stated in Chapter 1, it is necessary to combine suitable phosphors with a LED chip to produce white light, as phosphors play key roles in determining optical properties of white LED lamps, such as luminous ef”- ciency, chromaticity coordinate, color temperature, and lifetime. LED phosphors differ greatly from other types of phosphors in the excitation source. The excitation source is ultraviolet (350-410 nm) or blue (440-470 nm) LEDs for LED phosphors, whereas it is 254 nm, electron beam, and x-rays for lamp, CRT, and x-ray phosphors, respectively. This difference leads to different requirements for materials design and luminescent properties of LED phosphors, and also leads to the conclusion that other types of phosphors cannot be directly used as LED phosphors without any modi”cations. This means that it is necessary to search for and develop novel and suitable host crystals for LED phosphors, or to modify the currently available lamp phosphors.