  Just as parallel threebranes on a smooth manifold are related to string
theory on $AdS_5\times {\bf S}^5$, parallel threebranes near a conical
singularity are related to string theory on $AdS_5\times X_5$, for a suitable
$X_5$. For the example of the conifold singularity, for which $X_5=(SU(2)\times
SU(2))/U(1)$, we argue that string theory on $AdS_5\times X_5$ can be described
by a certain ${\cal N}=1$ supersymmetric gauge theory which we describe in
detail.
