  We show that with the help of a suitable coupling between dark energy and
cold dark matter it is possible to reproduce any scaling solution $\rho
_{X}\propto \rho_{M}a^{\xi}$, where $\rho_{X}$ and $\rho_{M}$ are the densities
of dark energy and dark matter, respectively. We demonstrate how the case $\xi
= 1$ alleviates the coincidence problem. Future observations of supernovae at
high redshift as well as quasar pairs which are planned to discriminate between
different cosmological models will also provide direct constraints on the
coupling between dark matter and dark energy.
