Interactions and Instabilities in Cosmology's Dark Sector
  I consider couplings between the dark energy and dark matter sectors. I
describe how the existence of an adiabatic regime, in which the dark energy
field instantaneously tracks the minimum of its effective potential, opens the
door for a catastrophic instability. This {\it adiabatic instability} tightly
constrains a wide class of interacting dark sector models. This talk was
presented at, and will appear in the proceedings of the DPF-2009 conference.
