  We investigate the field dependence of the gauge couplings of $N=1$ string
vacua from the point of view of the low energy effective quantum field theory.
We find that field-theoretical considerations severely constrain the form of
the string loop corrections; in particular, the dilaton dependence of the gauge
couplings is completely universal at the one-loop level. The moduli dependence
of the string threshold corrections is also constrained, and we illustrate the
power of such constraints with a detailed discussion of the orbifold vacua and
the $(2,2)$ (Calabi-Yau) vacua of the heterotic string.
