  I summarize perturbative and non-perturbative field theory tests of the
holographic correspondence between type IIB superstring on AdS_5xS^5 and N=4
SYM theory. The holographic duality between D-instantons and YM instantons is
briefly described. Non renormalization of two- and three-point functions of
CPO's and their extremal and next-to-extremal correlators are then reviewed.
Finally, partial non-renormalization of four-point functions of lowest CPO's is
analyzed in view of the interpretation of short distance logarithmic behaviours
in terms of anomalous dimensions of unprotected operators.
