Abstract. The discovery of the accelerating expansion of the Universe is motivating an impressive amount of theoretical and observational activity. After briefly reviewing the problems and challenges of "Dark Energy", I will focus on recent and ongoing works where a unifying description of dark energy and modified gravity is proposed, that is inspired by the Effective Field Theory (EFT) of Inflation formalism. By extending such a framework to late time cosmology one can write the most general action for cosmological perturbations in the presence of an additional scalar degree of freedom. I will focus on a few operators that are quadratic in the perturbations and which appear in non-minimally coupled scalar-tensor gravity and "Galileon" theories and describe the mixing between gravity and the scalar degree of freedom that such operators produce. I will also discuss the present observational constraints and forecasts for future experiments such as EUCLID.