  We consider the low-energy collective excitations at finite temperature of
Bose--Einstein condensed gases (and liquids as well). A most general
model-independent effective Lagrangian is written down according to a
prescription obtained from the breakdown of the global symmetry U(1). To show
how the theory predicts easily, we derive the momentum and temperature
dependence of the damping of excitations by means of power counting as an
example.
