I will present  an effective  description of  dark energy/modified gravity models involving a single scalar field.  It is based on a 3+1 decomposition of space-time with respect to uniform scalar field hypersurfaces. The advantage of this approach is that it can describe in the same language a vast number of existing models, including quintessence, F(R) gravity and Horndeski  theories. It also includes scalar-tensor theories “beyond Horndeski”, which do not appear to suffer from ghost-like instabilities in contrast to previous belief. This approach provides a unified treatment of linear cosmological perturbations, which generically turn out to be characterized by only five time-dependent functions. This gives an economic and systematic way to confront theoretical models with cosmological observations.