The discovery of the gravitational-wave (GW) source GW150914 with the Advanced LIGO detectors provides thefirst observational evidence for the existence of binary black hole (BH) systems that inspiral and merge within theage of the universe. Such BH mergers have been predicted in two main types of formation models, involvingisolated binaries in galactic fields or dynamical interactions in young and old dense stellar environments. Themeasured masses robustly demonstrate that relatively “heavy” BHs (25 M) can form in nature. This discoveryimplies relatively weak massive-star winds and thus the formation of GW150914 in an environment with ametallicity lower than about 1/2 of the solar value. The rate of binary-BH (BBH) mergers inferred from theobservation of GW150914 is consistent with the higher end of rate predictions (1 Gpc−3 yr−1) from both types offormation models. The low measured redshift (z 0.1) of GW150914 and the low inferred metallicity of thestellar progenitor imply either BBH formation in a low-mass galaxy in the local universe and a prompt merger, orformation at high redshift with a time delay between formation and merger of several Gyr. This discoverymotivates further studies of binary-BH formation astrophysics. It also has implications for future detections andstudies by Advanced LIGO and Advanced Virgo, and GW detectors in space.