It is well known that over 80 years ago, the concept of blackbody radiation came into existence and it would be used to illuminate our environment. After continuous efforts by scientists and researchers, lamps, TV sets, monitors, and medical scanners were developed and are now used everywhere. Solid inorganic luminescent materials play an important role in the development of this display and lighting technology. Currently, research in this area has been conducted for almost a hundred years, but the search for new phosphor materials continues. Phosphors are defined as solid luminescent materials that have the ability to absorb incident light (in the near ultraviolet [UV] or visible region) and then to emit light in a longer wavelength range (visible to near infrared [NIR]). Generally, a xenon flash lamp is used as the excitation source (i.e., the process of photoluminescence), or it may be an incident beam of electrons (a process called cathodoluminescence). The phosphor is composed of an inert divalent or trivalent host lattice doped with a rare earth-transition series element. The activator impurity ions typically have a 3d or 4f electron in their outermost shell, which is then excited under a UV or visible light source. In the luminescence process, the activator absorbs the incident energy and goes to a higher state, where relaxation occurs. Subsequently, it emits a photon and returns to the lower state. The emission originates from the phosphor when it is doped with divalent or trivalent impurity ions, so that it forms different energy levels and shows UV and visible emission. The phosphor efficiency depends on the amount of light absorbed and the amount of light emitted, and for most of the ideal phosphors, this ratio is considered to be unity. Other properties associated with ideal phosphors are thermal stability, color quality, color rendering index (CRI), and so on. During the phosphor relaxation process, part of the energy is lost due to nanoradiative relaxation of photons. This is the important factor that affects the luminous efficiency of the phosphor. So, for better 46luminous properties, there is a need for reduced nonradiative loss. At present, there are very few phosphor materials used on a commercial level for the fabrication of white light–emitting diodes (LEDs), compact fluorescent lights (CFLs), and display devices. Blue-, green-, yellow-, and red-emitting phosphors are the most used primary phosphors in many display devices. With a preferred combination of these, we can obtain a white emission. Thus, the application of phosphors may change according to their photoluminescence and related properties, which are found to be close to the ideal phosphor. In the third generation of photovoltaic (PV) technology, NIR-emitting phosphors were experimentally and practically proposed by many researchers to enhance the light conversion efficiency of solar cell devices. Hence, phosphor research is a leading field for researchers, having better technological advantages, such as energy-saving and energy-conversion capacity. In the next section, we will discuss some important terms that are related to luminescence, phosphors and their mechanisms, rare earth-transition spectroscopy, and so on.