  We present a supersymmetric recursion relation for tree-level scattering
amplitudes in N=4 super Yang-Mills. Using this recursion relation, we prove
that the tree-level S-matrix of the maximally supersymmetric theory is
covariant under dual superconformal transformations. We further analyse the
consequences that the transformation properties of the trees under this
symmetry have on those of the loops. In particular, we show that the
coefficients of the expansion of generic one-loop amplitudes in a basis of
pseudo-conformally invariant scalar box functions transform covariantly under
dual superconformal symmetry, and in exactly the same way as the corresponding
tree-level amplitudes.
