In recent years the discovery of the late-time cosmic acceleration triggered the creation of a multitude of theories, which modify General Relativity, in order to give a satisfactory explanation to this phenomenon. Now, with the wealth of current and upcoming observational data, it becomes imperative to create effcient ways to put these theories to the test, both in a model dependent and independent way. This led to the creation of the Effective Field Theory of Dark Energy and Modied Gravity which provides a framework to accomplish this.

In this talk I will introduce the EFT of DE/MG and comment on how it can be used in a model dependent and independent way. Moreover I will exhibit its potential to constrain the landscape of theories by extensively studying the theoretical stability conditions (ghost, gradient and tachy-onic/Jeans) of modied gravity theories in the presence of matter fluids. Naturally I will discuss about the differences in the parameters space of such theories when gravity is coupled to matter fields and in vacuum. Finally I will conclude by commenting on how these results can be employed in cosmological tools, such as EFTCAMB.