  We describe the influence of environmental noise on LIGO detectors in the
sixth science run (S6), from July 2009 to October 2010. We show results from
experimental investigations testing the coupling level and mechanisms for
acoustic, electromagnetic/magnetic and seismic noise to the instruments. We
argument the sensors' importance for vetoes of false positive detections,
report estimates of the noise sources' contributions to the detector
background, and discuss the ways in which environmental coupling should be
reduced in the LIGO upgrade, Advanced LIGO.
