  This Thesis discusses a number of issues related to the problem of tadpoles
and vacuum redefinitions that the breaking of supersymmetry brings about in
String Theory. The idea pursued here is to try to formulate the theory in a
``wrong'' vacuum (the vacuum that one naively identifies prior to the
redefinitions) and, gaining some intuition from some simpler field theory
settings, try to set up a calculational scheme for vacuum redefinitions in
String Theory. This requires in general complicated resummations, but some
simpler cases can be identified. This is true, in principle, for models with
fluxes, where tadpoles can be perturbatively small, and for the one-loop
threshold corrections, that in a large class of models (without rotated branes)
remain finite even in the presence of tadpoles. The contents of the Thesis
elaborate on those of hep-th/0410101, but include a number of additions,
related to the explicit study of a quartic potential in Field Theory, where
some subtleties were previously overlooked, and to the explicit evaluation of
the one-loop threshold corrections for a number of string models with broken
supersymmetry.

High Energy Physics - Theory