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The post-Minkowskian limit and gravitational wave solutions for generalfourth-order gravity theories are discussed. Specifically, we consider aLagrangian with a generic function of curvature invariants $f(R,R_{\alpha\beta}R^{\alpha\beta},R_{\alpha\beta\gamma\delta}R^{\alpha\beta\gamma\delta})$. It is well known thatwhen dealing with General Relativity such an approach provides masslessspin-two waves as propagating degree of freedom of the gravitational fieldwhile this theory implies other additional propagating modes in the gravityspectra. We show that, in general, fourth order gravity, besides the standardmassless graviton is characterized by two further massive modes with afinite-distance interaction. We find out the most general gravitational wavesolutions in terms of Green functions in vacuum and in presence of mattersources. If an electromagnetic source is chosen, only the modes induced by$R_{\alpha\beta}R^{\alpha\beta}$ are present, otherwise, for any $f(R)$ gravitymodel, we have the complete analogy with tensor modes of General Relativity.Polarizations and helicity states are classified in the hypothesis of planewave.

Post-Minkowskian Limit and Gravitational Waves solutions of Fourth Order Gravity: a complete study