  The emergence of an effective field theory out of equilibrium is studied in
the case in which a light field --the system-- interacts with very heavy fields
in a finite temperature bath. We obtain the reduced density matrix for the
light field, its time evolution is determined by an effective action that
includes the \emph{influence action} from correlations of the heavy degrees of
freedom. The non-equilibrium effective field theory yields a Langevin equation
of motion for the light field in terms of dissipative and noise kernels that
obey a generalized fluctuation dissipation relation. These are completely
determined by the spectral density of the bath which is analyzed in detail for
several cases. At $T=0$ we elucidate the effect of thresholds in the
renormalization aspects and the asymptotic emergence of a local effective field
theory with unitary time evolution. At $T\neq 0$ new "anomalous" thresholds
arise, in particular the \emph{decay} of the environmental heavy fields into
the light field leads to \emph{dissipative} dynamics of the light field. Even
when the heavy bath particles are thermally suppressed this dissipative
contribution leads to the \emph{thermalization} of the light field which is
confirmed by a quantum kinetics analysis. We obtain the quantum master equation
and show explicitly that its solution in the field basis is precisely the
influence action that determines the effective non-equilibrium field theory.
The Lindblad form of the quantum master equation features \emph{time dependent
dissipative coefficients}. Their time dependence is crucial to extract
renormalization effects at asymptotically long time. The dynamics from the
quantum master equation is in complete agreement with that of the effective
action, Langevin dynamics and quantum kinetics, thus providing a unified
framework to effective field theory out of equilibrium.
