Similar to electro-magnetic waves which are generated by accelerated charges, gravitational waves are generated every time a mass is accelerated. Contrary to electro-magnetic waves, gravitational waves penetrate everything and travel undisturbed through the universe. Observing gravitational waves will allow us to observe processes deep inside the centers of galaxies where no electro-magnetic radiation escapes unscattered. It will allow us to study gravity in strong fields, see small to medium size black holes being swallowed by their super-massive relatives, and observe the evolution of black hole binaries and mergers on mass scales ranging from solar mass galactic black holes to super-massive black holes in the centers of colliding galaxies. I will give an overview of the potential sources of gravitational waves and discuss current and future ground-based and space-based gravitational wave detectors.