Surface plasmon-polaritons (or plasmons) are collective excitations of the conduction electrons and the electromagnetic field on the surface of such good metals as gold and silver. The wavelength of a plasmon is considerably shorter than the wavelength of a free space photon near some characteristic frequency, which is located in the visible range. This frequency is called the frequency of surface plasmon resonance. When plasmonic devices are operated near this frequency the diffraction limit of optical field localization may be shifted down to the nanometer scale. As a result, optical energy may be guided by nanometer-scale plasmonic waveguides, and plasmon-based microscopy may potentially be developed, which would allow direct visualization of individual viruses and molecules. In addition, strong concentration of optical energy facilitates nonlinear interactions of surface plasmons, which opens new possibilities for making active nanophotonics devices. Potentially, plasmonics may combine the best features of photonics and electronics (the small size of electronics and the large data carrying ability of photonics). Thus, plasmonics may become "the next big thing" in nanotechnology.