Abstract：The first detection of a gravitational wave signal from a binary neutron star merger, GW 170817, was also promptly accompanied by an electromagnetic counterpart in the form of a short duration gamma-ray burst, GRB 170817A. A huge global effort of multi-wavelength follow-up observations has led to the detection of kilonova emission in the optical, UV and NIR, as well as long-lived X-ray to radio afterglow emission. This unique event has a wide range of implications ranging from constraints on the neutron star equation of state and maximal mass, through the important role of such binary mergers in r-process nucleosynthesis in the universe, to the type of remnant that was produced, and the properties of the outflows that are produced in these mergers.  This talk will first briefly outline the constraints on the type of compact remnant (black hole or massive neutron star) that was left in its aftermath, and will then focus on what can be learned from this event about the properties of the outflow that powered the prompt gamma-ray emission and the afterglow emission.