  The Cosmic Infrared Background (CIB) peaks in the Far-Infrared (FIR), and its
Spectral Energy Distribution (SED) is now well constrained. Thanks to recent
facilities and Spitzer, the populations contributing to the CIB are being
characterized: the dominant galaxy contributions to the FIR CIB are Luminous IR
galaxies (LIRGs) at 0.5<z<1.5 and, to the submm CIB, Ultra-LIRGs at z>2. These
populations of galaxies experience very high rates of evolution with redshift.
Because of confusion, the CIB is (and will remain in some domains) partially
resolved and its contributing galaxies SEDs are not well constrained. We
discuss all these aspects and show how confusion limits Spitzer observations,
and how to overcome it in order to study the unresolved part of the CIB.
