In-In correlators are the natural quantities in settings like cosmology or in non-equilibrium situations when the interest is not in scattering matrix elements but rather expectation values. The talk, after making clear the equivalence between the In-In and In-Out correlation functions, will describe a way to write down an effective field theory (EFT) description for In-In correlators in terms of the more familiar In-Out formalism routinely used for S-matrix calculations.