LIGO’s gravitational waves (GWs) detection has inaugurated an era to test the foundations of gravity. This includes also probing the nature of Dark Energy. Theories explaining the present acceleration of the Universe beyond the cosmological constant typically require adding extra gravitational degrees of freedom. This can lead to distinct signatures in the propagation of GWs. I will review current tests of gravity over different scales and regimes, and examine different frameworks describing dynamical dark energy. Then, focusing on the case of only one additional degree of freedom, I will present how the speed of GWs could be used to place severe constraints on generic scalar-tensor theories of gravity. I will describe under which circumstances an anomalous propagation speed can arise, and how it could be measured in the near future. If there is a small deviation from the speed of light, the delay between the GW and any electromagnetic counterpart will run beyond human time scales. Still, this measurement could be achieved with LISA using eclipsing white dwarf binaries. This test will either eliminate many contender models for cosmic acceleration or wreck a fundamental pillar of general relativity.