Following a major upgrade, the two advanced detectors of the Laser Interferometer Gravitational-waveObservatory (LIGO) held their first observation run between September 2015 and January 2016. With astrain sensitivity of 10^(−23)/(Hz)^(1/2) at 100 Hz, the product of observable volume and measurement timeexceeded that of all previous runs within the first 16 days of coincident observation. On September 14,2015, the Advanced LIGO detectors observed a transient gravitational-wave signal determined to be thecoalescence of two black holes [B. P. Abbott et al., Phys. Rev. Lett. 116, 061102 (2016)], launching the eraof gravitational-wave astronomy. The event, GW150914, was observed with a combined signal-to-noiseratio of 24 in coincidence by the two detectors. Here, we present the main features of the detectors thatenabled this observation. At full sensitivity, the Advanced LIGO detectors are designed to deliver anotherfactor of 3 improvement in the signal-to-noise ratio for binary black hole systems similar in mass toGW150914.