General relativity predicts that moving massive objects generate gravitational waves, ripples in spacetime that propagate at the speed of light. Direct detection of gravitational waves was first announced in February of 2016. Vitale reviews the scientific results of gravitational wave astronomy over the subsequent 5 years. About 50 events have been detected, mostly the mergers of binary black holes. The mass distribution of those events is unlike previously known black holes and constrains the evolution of massive stars. A binary neutron star merger was detected in both gravitational waves and electromagnetic radiation, a form of multi-messenger astrophysics. Tests of general relativity and cosmological measurements have also been performed. Science, abc7397, this issue p. eabc7397 A Review highlights how direct detections of gravitational waves have improved our understanding of black holes and neutron stars. Gravitational waves are ripples in spacetime generated by the acceleration of astrophysical objects; a direct consequence of general relativity, they were first directly observed in 2015. Here, I review the first 5 years of gravitational-wave detections. More than 50 gravitational-wave events have been found, emitted by pairs of merging compact objects such as neutron stars and black holes. These signals yield insights into the formation of compact objects and their progenitor stars, enable stringent tests of general relativity, and constrain the behavior of matter at densities higher than that of an atomic nucleus. Mergers that emit both gravitational and electromagnetic waves probe the formation of short gamma-ray bursts and the nucleosynthesis of heavy elements, and they measure the local expansion rate of the Universe.