General relativity is a physical theory on gravitation that is based on Riemannian/Lorentzian geometry or, more precisely, on the physics and geometry of the Einstein equations. From the linearized equations, Einstein got the idea of gravitational waves. However, the nature of gravitational radiation was only clarified half a century later by the work of physicists Bondi, Sachs, Penrose etc. I will introduce the theory, Bondi-Sachs formalism, behinds it.