  A statistical field theory for an inhomogeneous liquid, a planar liquid/vapor
interface, is devised from first principles. The grand canonical partition
function is represented via a Hubbard-Stratonovitch tranformation leading,
close to the critical point, to the usual $\phi^4$ scalar field theory which is
then rigorously considered at the one-loop level. When further simplified it
yields the well-known capillary wave theory without any ad hoc phenomenological
parameter. Internal coherence of the one-loop approximation is discussed and
good overall qualitative agreement with recent numerical simulations is
stressed.
