In this talk I will discuss how it is possible to use string theory to compute supergravity scattering amplitudes. I will focuss on the particular example of N=4 supergravity where I will show that the 3-loop divergence in the four graviton amplitude vanishes due to a non-renormalisation theorem in heterotic string. I will then discuss the particular issue of the decoupling of the vector multiplets in N=4 supergravity models, that make the definition of a unique theory of pure N=4 supergravity ambiguous.