The optical properties of metallic NP are determined by the surface plasmon resonance phenomenon. The SPR is caused by the coherent motion of the conduction band electrons from one surface of the particle to the other, upon interaction with an electromagnetic field. The decrease in size below the electron mean free path (distance the electron travels between scattering collisions with the lattice centers) gives rise to intense absorption in the UV-vis range. Optical excitation of the SPR gives rise to the surface plasmon absorption. The surface plasmon absorption depends on the size and the shape of the particles, and on the dielectric constant of the matrix. For small particles it depends also on the nature of the molecules adsorbed on the NP surface. Noble metals exhibit extremely weak photoluminescence in the bulk, assigned to recombination of the excited electrons with the holes within the d band. In metal NP the luminescence yield increases by as much as 6 orders of magnitude because of the so-called lightning rod effect: the incoming (exciting) and outcoming (emitted) electric fields are amplified by the plasmon resonances around the particles. Hence, the luminescence energy and efficiency are subjected to the size effect.