Multi-messenger gravitational wave (GW) astronomy has commenced with the detection of the binary neutron star merger GW170817 and its associated electromagnetic counterparts. The almost coincident observation of both signals places an exquisite bound on the GW speed |cg/c − 1| ≤ 5 · 10 −16 . We use this result to probe the nature of dark energy (DE), showing that a large class of scalar-tensor theories and DE models are highly disfavored. As an example we consider the covariant Galileon, a cosmologically viable, well motivated gravity theory which predicts a variable GW speed at low redshift. Our results eliminate any late-universe application of these models, as well as their Horndeski and most of their beyond Horndeski generalizations. Three alternatives (and their combinations) emerge as the only possible scalar-tensor DE models: 1) restricting Horndeski's action to its simplest terms, 2) applying a conformal transformation which preserves the causal structure and 3) compensating the different terms that modify the GW speed (to be robust, the compensation has to be independent on the background on which GWs propagate). Our conclusions extend to any other gravity theory predicting varying cg such as Einstein-Aether, Hořava gravity, Generalized Proca, TeVeS and other MOND-like gravities. [6]. In this letter we present the implications that this measurement has for the nature of dark energy (DE) and tests of General Relativity (GR).The present cosmic acceleration is probably one of the greatest challenges in modern physics. Leaving the theoretical fine tuning issues aside [7], a cosmological constant is the leading candidate to explain this acceleration since it is fully consistent with observations [8]. Alternative scenarios that explain DE dynamically require either additional degrees of freedom (beyond the massless spin-2 field of GR) or a low-energy violation of fundamental principles such as locality [9]. The extremely low energy scale for DE requires additional degrees of freedom to be hidden on small scales by a screening mechanism [10], which also suppresses their rate of emission as additional gravitational wave polarizations [11].New fields coupled to gravity can affect the propagation speed of the standard GW polarizations, as measured by GW170817 and its counterparts [12]. Anomalous GW speed can be used to test even screened theories, as signals from extra-galactic sources probe unscreened, cosmological scales. In addition, effects on GW propagation accumulate over the travel time of the signals, amplifying their magnitude and yielding an impressive sensitivity. GW astronomy is therefore the most powerful tool to test models that modify GW propagation.Some of the most interesting dark energy models predict an anomalous GW speed and are ruled out by GW170817. These include cosmologically viable, screened and self-accelerating models like the covariant Galileon [13,14], or proposals to solve the cosmological constant problem like the Fab-four [15]. We will describe the implications of GW170817 on thes...