Recent studies on the optical properties, in particular, luminescence, of a variety of doped semiconductor nanoparticles are reviewed. The effects of quantum confinement, temperature and pressure on luminescent properties are discussed. In addition, electroluminescence, cathodoluminescence, magnetoluminescence and related applications involving doped semiconductor nanoparticles are presented. A new phenomenon, upconversion luminescence of doped nanoparticles, is reviewed and its potential applications are discussed. While more research efforts are necessary in order to fully understand the fundamentals and explore the great technological potential behind doped nanoparticles, recent results already show that this is a new and exciting field with applications in many areas. In particular, the emerging field of "spintronics", where spin states are exploited in analogy to conventional electronic states, is discussed and the advantages of using doped semi-conductor nanoparticles are elucidated.