  Brane world scenarios offer a way of ensuring that a Poincare invariant four
dimensional world can emerge, without fine tuning, as a solution to the
equations of motion of an effective action. We discuss the different ways in
which this happens, and point out that the underlying reason is that there is a
contribution to the effective cosmological constant which is a constant of
integration, that maybe adjusted to ensure a flat space solution. Basically
this is an old idea revived in a new context and we speculate that there may be
string scenarios that provide a concrete realization of it. Finally we discuss
to what extent this is a solution to the cosmological constant problem.
