For hundreds of years since Galileo’s times, the sky was being observed with optical telescopes that looked at celestial objects either by the visible light that they emitted or by the light that was scattered off them. With advances in technology, the universe began to be observed using different wavelengths of the electromagnetic spectrum. Observational windows in radio waves, infrared and ultraviolet rays, X-rays and gamma rays opened up, and many terrestrial and space-based instruments have enabled scientists to gain new insights into the working of the universe. Neutrino astronomy, which looks at neutrinos from solar and extrasolar sources, is another window to the universe that has opened up in the last few decades. Now, the discovery of gravitational waves opens up an entirely new paradigm in observational astronomy.