Brane world scenarios and the cosmological constant

Brane world scenarios offer a way of ensuring that a Poincare invariant fou r-dimensional world can emerge, without fine tuning, as a solution to the e quations 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 consta nt of integration, that maybe adjusted to ensure a flat space solution. Bas ically this is an old idea revived in a new context and we speculate that t here may be string scenarios that provide a concrete realization of it. Fin ally we discuss to what extent this is a solution to the cosmological const ant problem. (C) 2001 Published by Elsevier Science B.V.