  The sensitivity of the Advanced LIGO detectors to gravitational waves can be
affected by environmental disturbances external to the detectors themselves.
Since the transition from the former initial LIGO phase, many improvements have
been made to the equipment and techniques used to investigate these
environmental effects. These methods have aided in tracking down and mitigating
noise sources throughout the first three observing runs of the advanced
detector era, keeping the ambient contribution of environmental noise below the
background noise levels of the detectors. In this paper we describe the methods
used and how they have led to the mitigation of noise sources, the role that
environmental monitoring has played in the validation of gravitational wave
events, and plans for future observing runs.
