  We investigate the role of a possible coupling of dark matter and dark
energy. In particular, we explore the consequences of such an interaction for
the coincidence problem, i.e., for the question, why the energy densities of
dark matter and dark energy are of the same order just at the present epoch. We
demonstrate, that, with the help of a suitable coupling, it is possible to
reproduce any scaling solution $\rho_X \propto \rho_M a^\xi$, where $a$ is the
scale factor of the Robertson-Walker metric and $\xi$ is a constant parameter.
$\rho_X$ and $\rho_M$ are the densities of dark energy and dark matter,
respectively. Furthermore, we show that an interaction between dark matter and
dark energy can drive the transition from an early matter dominated era to a
phase of accelerated expansion with a stable, stationary ratio of the energy
densities of both components.
