Dark energy - a homogeneously distributed cosmic energy density - seems to explain various cosmological observations. The anisotropies in the microwave background radiation, the formation of structure and the late time acceleration of the Hubble expansion fit into a consistent picture. We discuss quintessence - a dynamical form of dark energy and possible signatures distinctive from a cosmological constant. Particular emphasis is paid to the possible presence of Early Dark Energy and on an explanation why Dark Energy becomes important in a recent cosmological epoch. Quintessence could be related to a new "fundamental" macroscopic force and induce a small time variation of fundamental constants.