The Gamma Ray Opacity of the Universe -- Indirect Measurements of the
  Extragalactic Background Light
  Indirect constraints on the intensity of the Extragalactic Background Light
(EBL) were provided by recent studies of extragalactic sources emitting sub-TeV
to multi-TeV photons. These constraints are provided thanks to the absorption
of gamma rays by soft photons from the EBL (UV/optical/IR) via pair production
by gamma - gamma interactions. This paper provides an overview of recent
results that have led to substantially reduced uncertainties on the EBL
intensity over a wide range of wavelengths from 0.1 to 15 micron.
