Abstract: The discovery of the binary neutron star inspiral GW170817 was one of the most important events in astronomy. It not only brought us unprecedented knowledge about neutron stars, gamma-ray bursts, kilonovae and the astrophysical environment, but also provided a novel testbed for strong-field gravity. Many distinct aspects of Einstein's general relativity have been tested, including the speed of gravity, the mass of the graviton, the symmetry of spacetime, and the radiation and polarization modes of gravitational waves. No deviation from general relativity has been found.