  By integrating out the heavy fields in type II or heterotic string field
theory one can construct the effective action for the light fields. This
effective theory inherits all the algebraic structures of the parent theory and
the effective action automatically satisfies the Batalin-Vilkovisky quantum
master equation. This theory is manifestly ultraviolet finite, has only light
fields as its explicit degrees of freedom, and the Feynman diagrams of this
theory reproduce the exact scattering amplitudes of light states in string
theory to any arbitrary order in perturbation theory. Furthermore in this
theory the degrees of freedom of light fields above certain energy scale are
also implicitly integrated out. This energy scale is determined by a particular
parameter labelling a family of equivalent actions, and can be made arbitrarily
low, leading to the interpretation of the effective action as the Wilsonian
effective action.
