On August 17, 2017, the Advanced LIGO and Advanced Virgo gravitational-wave detectors observed a binary neutron star inspiral, known as GW170817. This event, with a combined signal-to-noise ratio of 32.4 and a false-alarm rate of less than one per 80,000 years, was the loudest gravitational-wave signal ever detected. The source was localized to a sky region of 28 square degrees and had a luminosity distance of 40 ± 8 megaparsecs, making it the closest and most precisely localized gravitational-wave signal to date. The association with the gamma-ray burst GRB 170817A, detected 1.7 seconds after the coalescence, confirmed the hypothesis that neutron star mergers produce short gamma-ray bursts. Subsequent electromagnetic observations across the spectrum further supported the interpretation of this event as a binary neutron star merger. This joint gravitational and electromagnetic observation provides valuable insights into astrophysics, dense matter, gravitation, and cosmology.On August 17, 2017, the Advanced LIGO and Advanced Virgo gravitational-wave detectors observed a binary neutron star inspiral, known as GW170817. This event, with a combined signal-to-noise ratio of 32.4 and a false-alarm rate of less than one per 80,000 years, was the loudest gravitational-wave signal ever detected. The source was localized to a sky region of 28 square degrees and had a luminosity distance of 40 ± 8 megaparsecs, making it the closest and most precisely localized gravitational-wave signal to date. The association with the gamma-ray burst GRB 170817A, detected 1.7 seconds after the coalescence, confirmed the hypothesis that neutron star mergers produce short gamma-ray bursts. Subsequent electromagnetic observations across the spectrum further supported the interpretation of this event as a binary neutron star merger. This joint gravitational and electromagnetic observation provides valuable insights into astrophysics, dense matter, gravitation, and cosmology.