  We argue that traditional methods of compactification of string theory make
it very difficult to understand how the cosmological constant becomes zero.
String inspired models can give zero cosmological constant after fine tuning
but since string theory has no free parameters it is not clear that this is
allowed. Brane world scenarios on the other hand while they do not answer the
question as to why the cosmological constant is zero do actually allow a choice
of integration constants that permit flat four space solutions. In this paper
we discuss gauged supergravity realizations of such a world. To the extent that
this starting point can be considered a low energy effective action of string
theory (and there is some recent evidence supporting this) our model may be
considered a string theory realization of this scenario.
