Abstract.  A new exploration of the Universe has recently started through multi-messenger observations including gravitational-waves. The detection of gravitational-waves and multi-wavelength electromagnetic emissions from a binary neutron star merger has demonstrated the enormous potential of multi-messenger astronomy to investigate the most energetic transients in the sky, probing relativistic astrophysics, nuclear physics, nucleosynthesis, and cosmology. The talk will give an overview of the astrophysical implications of gravitational-wave and multi-messenger observations, the prospects and challenges of the current and future gravitational-wave detectors, such as Einstein Telescope which is expected to undergo a revolution for transient (astro)physics.