Linear perturbation theory is at the moment the only analytical tool to study properties of dark energy and modified gravity models beyond the background level. A very powerful framework, which can be applied to generic classes of models is the Equation of State (EoS) approach. According to it, linear perturbations can be completely parametrized by defining two gauge-invariant quantities which represent the equations of state of the model. The EoS approach applies to scalar, vector and tensor perturbations as well and is based on the fluid approach. In this talk I will give an overview of the EoS approach and of its applications to two specific models, f(R) and Generalized Einstein-Aether, and how it could help us evading the gravitational waves constraints on modified gravity models.