Last year, the LIGO Scientific Collaboration and the Virgo Collaboration announced the first detection of a gravitational wave. A century after the fundamental predictions of Einstein, the first direct observation of a binary black hole system merging to form a single black hole was made. The observations provide unique access to the properties of spacetime at extreme curvatures: the strong-field and high-velocity regime. It allows unprecedented tests of general relativity for the nonlinear dynamics of highly disturbed black holes. LIGO and Virgo realized a global interferometer network, and more detections were made, including a signal from a binary neutron star merger. The scientific impact of the various detections will be explained. In addition, key technological aspects will be addressed, such as the interferometric detection principle, optics, as well as sensors and actuators. Attention is paid to Advanced Virgo, the European detector near Pisa, which came online in 2017. We end with a discussion of the largest challenges in the field, including plans for the Einstein Telescope, a large underground observatory for gravitational-wave science.