On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. TheFermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a skyregion of 31 deg2 at a luminosity distance of 40 88-+ Mpc and with component masses consistent with neutron stars. Thecomponent masses were later measured to be in the range 0.86 to 2.26 Me. An extensive observing campaign waslaunched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now withthe IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independentlydetected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Earlyultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed aredward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered atthe transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likelyarise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. Noultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches.These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars inNGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by theradioactive decay of r-process nuclei synthesized in the ejecta.