High-redshift ultra luminous infrared galaxies contribute the bulk of the cosmic IR background and are the best candidates for very massive galaxies in formation at z>1.5. It is necessary to identify the energy source for their huge luminosities, starburst or AGN activity, in order to correctly interpret the role of ULIRGs in galaxy evolution, and compute reliable estimates of their star formation rates, stellar masses, and accretion luminosities.