On 2017 August 17 the Advanced LIGO and Advanced Virgo detectors observed the merger of
a binary neutron star system. The Fermi-GBM and INTEGRAL SPI-ACS instruments indepen-
dently detected the short Gamma Ray Burst GRB 170817A with a delay of about 1.7 s. The
gravitational source was localized within a sky region of about 30 deg 2 and at a luminosity dis-
tance of 40 Mpc. The masses of the components were consistent with the known masses of
neutron stars. A worldwide observing campaign across the electromagnetic spectrum discovered
an optical transient (SSS17a/AT 2017gfo) in the elliptical galaxy NGC 4993. The first optical
detection was made less than 11 hours after the merger. The optical and infrared observations in
the first days showed the signature of a kilonova. The first X-ray and radio detection of the tran-
sient occurred at 9 and 16 days after the merger, respectively. The multi-messenger observations
suggest that GW170817 was produced by the merger of two neutron stars in NGC 4993, followed
by the short gamma ray burst GRB 170817A and by a kilonova whose emission was powered by
the radioactive decay of r-process nuclei synthesized in the ejecta. This review summarizes the
initial multi-messenger observations of GW170817.