  We present an overview of quantum noise in gravitational wave
interferometers. Gravitational wave detectors are extensively modified variants
of a Michelson interferometer and the quantum noise couplings are strongly
influenced by the interferometer configuration. We describe recent developments
in the treatment of quantum noise in the complex interferometer configurations
of present-day and future gravitational-wave detectors. In addition, we explore
prospects for the use of squeezed light in future interferometers, including
consideration of the effects of losses, and the choice of optimal readout
schemes.
