The optical properties of lanthanide (Ln3+) ions were recognized many years ago in several important elds of science and technology. Lanthanide-doped crystal, glasses, and bers have been used as optically active materials for compact solid-state laser crystals (Geusic et al. 1964; Moncorgé et al. 1999; Quarles et al. 1990), ber lasers (Digonnet 2001), TV tubes or lamp phosphors (Lakshmanan 2008), as well as IR quantum cutters (Esterowitz et al. 1968). The unique optical features of lanthanide-doped nanomaterials arise from the electronic conguration of optically active ions doped into the crystal matrix. The 4f-4f intra-congurational transitions are Laporte forbidden, and simultaneously the electrons at 4f energy levels are shielded by the lled higher orbitals. The consequences of forbidden character of electronic transitions observed in lanthanides are among others: low transition probability, and further long luminescence lifetimes, which vary between micro to milliseconds. These long decays facilitate ultrahigh sensitivity detection, since in time-resolved mode, the background free detection becomes possible. In addition, due to the weak interaction of electrons at 4f orbitals with the crystal eld of the host matrix, the absorption and emission lines are very narrow and occur in strictly dened spectral ranges, which on the other hand, enable the engineered design of multicolor emission and further multiplex detection.