  Hyperbolic metamaterials were originally introduced to overcome the
diffraction limit of optical imaging. Soon thereafter it was realized that
hyperbolic metamaterials demonstrate a number of novel phenomena resulting from
the broadband singular behavior of their density of photonic states. These
novel phenomena and applications include super resolution imaging, new stealth
technologies, enhanced quantum-electrodynamic effects, thermal
hyperconductivity, superconductivity, and interesting gravitation theory
analogues. Here we briefly review typical material systems, which exhibit
hyperbolic behavior and outline important applications of hyperbolic
metamaterials.
