Dark Matter and Dark Energy Interactions: Theoretical Challenges,
  Cosmological Implications and Observational Signatures
  Models where Dark Matter and Dark Energy interact with each other have been
proposed to solve the coincidence problem. We review the motivations underlying
the need to introduce such interaction, its influence on the background
dynamics and how it modifies the evolution of linear perturbations. We test
models using the most recent observational data and we find that the
interaction is compatible with the current astronomical and cosmological data.
Finally, we describe the forthcoming data sets from current and future
facilities that are being constructed or designed that will allow a clearer
understanding of the physics of the dark sector.
