  The origin of accelerating expansion of the Universe is one the biggest
conundrum of fundamental physics. In this paper we review vacuum energy issues
as the origin of accelerating expansion - generally called dark energy - and
give an overview of alternatives, which a large number of them can be
classified as interacting scalar field models. We review properties of these
models both as classical field and as quantum condensates in the framework of
non-equilibrium quantum field theory. Finally, we review phenomenology of
models with the goal of discriminating between them.
