  Attempts to solve Yang-Mills theory must eventually face the problem of
analyzing the theory at intermediate values of the coupling constant. In this
regime neither perturbation theory nor the gravity dual are adequate, and one
must consider the full string theory in the appropriate background. We suggest
that in some nontrivial cases the world sheet theory may be exactly solvable.
For the Green-Schwarz superstring on AdS_5 x S^5 we find an infinite set of
nonlocal classically conserved charges, of the type that exist in integrable
field theories.
