  In 2009-2010, the Laser Interferometer Gravitational-wave Observa- tory
(LIGO) operated together with international partners Virgo and GEO600 as a
network to search for gravitational waves of astrophysical origin. The
sensitiv- ity of these detectors was limited by a combination of noise sources
inherent to the instrumental design and its environment, often localized in
time or frequency, that couple into the gravitational-wave readout. Here we
review the performance of the LIGO instruments during this epoch, the work done
to characterize the de- tectors and their data, and the effect that transient
and continuous noise artefacts have on the sensitivity of LIGO to a variety of
astrophysical sources.
