In the past few years the Horndeski class of models has attracted considerable attention in the cosmology community, since it aims to be the most general scalar-tensor theory with second-order equations of motions on any background. After an introduction of the Horndeski type of Dark Energy/Modified Gravity, I will focus on the evolution of linear perturbations on a cosmological background. In particular, I will explain how to obtain a minimal description preserving the maximal freedom of these models. This is of particular interest since it separates the physical effects of the theory and it allows to constrain a large class of models at once. Finally, I shall show some results of the Hi-Class project, an implementation of the Horndeski type of gravity in the public available Class Boltzmann code.