During the first decades after Einstein had developed his Theory of General Relativity, the main effort was to understand the theory and verify it experimentically. Meanwhile Genral Relativity is one of the experimentally best confirmed theories and has become a powerful tool for the investigation of cosmic processes where strong gravitational fields are involved. This book contains 16 contributions from well-known experts giving a broad overview for non-specialists who want to learn how to purely academic issues like gravitational wave detectors are now put into reality.