Brane world scenarios and the cosmological constant

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