  In this letter we reflect on the propagation of gravitational waves in
alternative theories of gravity, which are typically formulated using extra
gravitational degrees of freedom in comparison to General Relativity. We
propose to understand that additional structure as forming a diagravitational
medium for gravitational waves characterized by a refractive index.
Furthermore, we shall argue that the most general diagravitational medium has
associated an anisotropic dispersion relation. In some situations a refractive
index tensor, which takes into account both the deflection of gravitational
waves due to the curvature of a non-flat spacetime and the modifications of the
general relativistic predictions, can be defined. The most general media,
however, entail the consideration of at least two independent tensors.
