  The extragalactic background light (EBL) is the relic emission of all
processes of structure formation in the Universe. About half of this
background, called the Cosmic Infrared Background (CIB) is emitted in the
8-1000 microns range, and peaks around 150 microns. It is due to the dust
reemission from star formation processes and AGN emission. The CIB spectral
energy distribution (SED) constraints the models of star formation in the
Universe. It is also useful to compute the opacity of the Universe to the TeV
photons.
  We present the different types of measurements of the CIB and discuss their
strengths and weaknesses.
  1. The absolute SED was measured by COBE, and by other experiments. These
measurements are limited by the accuracy of the component separation, i.e. the
foreground subtraction.
  2. Robust lower limits are determined from the extragalactic number counts of
infrared galaxies. These lower limits are very stringent up to 100 microns. At
larger wavelengths, the rather low angular resolution of the instruments limits
strongly the depth of the number counts. The "stacking" method determines the
flux emitted at a given wavelength by a population detected at another
wavelength, and provides stringent lower limits in the sub-mm range. It is
complementary with other methods based on the statistical analysis of the map
properties like the P(D) analysis.
  3. Finally, upper limits can be derived from the high energy spectra of
extragalactic sources. These upper limits give currently good constraints in
the near- and mid-IR.
  Progress have been amazing since the CIB discovery about 15 years ago: the
SED is much better known, and most of it can be accounted for by galaxies
(directly or indirectly). Prospects are also exciting, with fluctuation
analysis with Planck&Herschel, and forthcoming missions.
