  Gravitational waves from binary black hole and neutron star mergers are being
regularly detected. As of 2021, ninety confident gravitational wave detections
have been made by the LIGO and Virgo detectors. Work is ongoing to further
increase the sensitivity of the detectors for the fourth observing run,
including installing some of the A+ upgrades designed to lower the fundamental
noise that limits the sensitivity to gravitational waves. In this review, we
will overview how the LIGO detectors work, including their optical
configuration and lock acquisition procedure, discuss the detectors'
fundamental and technical noise limits and compare to the current measured
sensitivity, and review the A+ upgrades currently being installed in the
detectors.
