  In this Chapter, we give a brief review of the state of the art of
theoretical and experimental studies of quantum fluids of light. Such systems
consist of ensembles of photons that acquire a finite mass from spatial
confinement or diffraction and finite binary interactions from the optical
nonlinearity of the optical medium. The peculiar properties of these fluids are
highlighted in comparison with standard condensed matter systems, with a
special emphasis on the novel possibilities that they offer for the generation,
the manipulation and the diagnostics of the fluid, as well as on their
intrinsically non-equilibrium and/or dynamical nature. Perspectives towards a
new generation of experiments on strongly correlated fluids of light and
towards opto-electronic applications are finally sketched.
