Since 1998 there is growing observational evidence for the present Universe to be in a state of accelerated expansion. Within the framework of Einstein’s theory of General Relativity this situation requires a dynamically dominating “exotic” matter component, characterized by a negative pressure which is called “Dark Energy” (DE). On the other hand, to explain the origin of cosmic structures, a substantial amount of pressureless “Dark Matter” (DM) is required. Most investigations in this field are phenomenological and rely on the assumption that DE and DM evolve independently, i.e., their energies are assumed to obey separate conservation laws. A coupling in the dark sector, however, will modify the evolution history of the Universe. Models with an interaction between DE and DM are also useful tools to address the “coincidence problem”, i.e., the question, why the energy densities of DM and DE are of the same order just at the present epoch. It is the purpose of this talk to discuss several aspects and possible consequences of such a coupling.