  Tadpoles accompany, in one form or another, all attempts to realize
supersymmetry breaking in String Theory, making the present constructions at
best incomplete. Whereas these tadpoles are typically large, a closer look at
the problem from a perturbative viewpoint has the potential of illuminating at
least some of its qualitative features in String Theory. A possible scheme to
this effect was proposed long ago by Fischler and Susskind, but incorporating
background redefinitions in string amplitudes in a systematic fashion has long
proved very difficult. In the first part of this paper, drawing from field
theory examples, we thus begin to explore what one can learn by working
perturbatively in a ``wrong'' vacuum. While unnatural in Field Theory, this
procedure presents evident advantages in String Theory, whose definition in
curved backgrounds is mostly beyond reach at the present time. At the field
theory level, we also identify and characterize some special choices of vacua
where tadpole resummations terminate after a few contributions. In the second
part we present a notable example where vacuum redefinitions can be dealt with
to some extent at the full string level, providing some evidence for a new link
between IIB and 0B orientifolds. We finally show that NS-NS tadpoles do not
manifest themselves to lowest order in certain classes of string constructions
with broken supersymmetry and parallel branes, including brane-antibrane pairs
and brane supersymmetry breaking models, that therefore have UV finite
threshold corrections at one loop.
