This section is a brief review of the existing hydrodynamic theories for granular "fluids". The theories slightly differ in the constitutive relations that express the fluxes (e.g. stress tensor and heat flow) as functions of the fields (density, velocity and granular temperature). Nevertheless all the theories assume the hydrodynamical limit (see paragraph 2.2.4), i.e. a separation of scales between macroscopic slow changes and microscopic rapid relaxation to equilibrium: moreover almost all the authors, in order to obtain a rigorous derivation of the transport coefficients, work in the Boltzmann-Grad limit, so that the Boltzmann equation, i.e. the Molecular Chaos, can be considered valid. Other differences are in the intensity of dissipation considered (i.e. the range of values of the restitution coefficient), which is reflected in the limits of validity of certain perturbative expansions.