How does open/closed duality work? In some non-critical (often topological) string theories we can answer this question exactly. We begin by reviewing the relevant geometry of the moduli space of open and closed Riemann surfaces. We then describe some simple matrix models and their precise interpretation as the open string duals to topological closed string theories. We attempt to draw general lessons about AdS/CFT.

References : hep-th/0312196, Fortsch.Phys.52:302-337 (2004)