  We report on the computation of the one-instanton contribution to the
16-point Green function of fermionic composite operators in N=4 Super YM
theory. The remarkable agreement, initially found in the case of an SU(2) gauge
group, with the one D-instanton contribution to the corresponding type IIB
superstring amplitude on $AdS_5\times S^5$ is reviewed here. The recent
extension by other authors of this result to any SU(N) gauge group and to
multi-instantons in the large N limit is briefly discussed. We also argue that
for the AdS/SCFT correspondence under consideration to work for any $N\geq 2$,
at least for some protected interactions, string effects should require a
truncation of the Kaluza--Klein spectrum for finite $S^5$ radius much in the
same way as worldsheet unitarity restricts the allowed isospins in SU(2) WZW
models. Finally, we briefly comment on the logarithmic behavior of some
four-point Green functions of scalar composite operators.
