On 2017 August 17 the Advanced LIGO and Advanced Virgo detectors detected for the first time the signal, GW170817, from the coalescence of a binary system of neutron stars (Abbott B.P. et al., Phys. Rev. Lett., 119 (2017) 161101). Exactly 1.7 s after the merger time (12:41:04 UTC) the Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst, GRB170817A (Abbott B.P. et al., Astrophys. J. Lett., 848 (2017) L13; L14). An extensive observing campaign involving more than 70 world-wide ground and space observatories was performed leading to the discovery of the counterpart signals across all the electromagnetic spectrum (Abbott B.P. et al., Astrophys. J. Lett., 848 (2017) L12). This observational campaign marks the birth of multi-messenger astronomy, which uses gravitational waves and electromagnetic emission. The collected multi-messenger data confirmed ten-year-old theoretical models. And at the same time, the richness of details of the taken data will require to develop new theory and to make other observations in the coming years to be interpreted