  The integrable structure of open--closed string theories in the $(p,q)$
conformal minimal model backgrounds is presented. The relation between the
$\tau$--function of the closed string theory and that of the open--closed
string theory is uncovered. The resulting description of the open--closed
string theory is shown to fit very naturally into the framework of the
$sl(q,{\rm C})$ KdV hierarchies. In particular, the twisted bosons which
underlie and organise the structure of the closed string theory play a similar
role here and may be employed to derive loop equations and correlation function
recursion relations for the open--closed strings in a simple way.
