In the previous chapter we determined the equilibrium equations for the director n(r) such that the total elastic energy is minimized. We shall now study the evolution of a (uniaxial) nematic taken out of thermodynamic equilibrium by an external perturbation. This is the field of nematodynamics, more complicated but also richer than conventional hydrodynamics, due to the coupling between the fields n(r) and v(r) (hydrodynamic velocity).