  We discuss in detail how to consistently impose boundary conditions in
quantum string cosmology. Since a classical time parameter is absent in quantum
gravity, such conditions must be imposed with respect to intrinsic variables.
Constructing wave packets for minisuperspace models from different tree-level
string effective actions, we explain in particular the meaning of a transition
between ``pre-big-bang" and ``post-big-bang" branches. This leads to a scenario
different from previous considerations.
