  Based on a generalization of the string theoretic concept of D-brane probe,
we propose a new approach to large N gauge theories which makes the holographic
properties manifest. For any gauge theory, we define from first principles an
effective action for a fixed number of "probe" D-branes in the presence of N
"background" D-branes on which the gauge theory lives. This effective action is
shown to encode all the information about the large N gauge theory. The
analysis of the planar diagram expansion which computes the effective action
yields a simple and generic mechanism explaining the emergence of holographic
space dimensions: the probe D-branes move in a higher dimensional dual
holographic space-time. The construction yields a new perspective on the notion
of bulk space-time locality and draws unexpected links with some aspects of the
't Hooft Abelian projection ideas. It also provides a new non-perturbative
approximation scheme, able to capture both the weak and strong coupling
regimes. We sketchily illustrate the basic ideas on a few examples, including
the pure four dimensional Yang-Mills theory.
