The energy input into the interstellar medium in Ultraluminous Infrared Galaxies (ULIRGs) is enormous, regardless of the nature of the power source. I discuss some of the major consequences for the structure and energetics of the ISM in these galaxies. Observationally, the column densities in the nuclear regions of ULIRGs are known to be very high, which makes distinguishing starbursts from AGN quite difficult. The level of energy and momentum injection means that the pressure in the ISM must be extremely high, at least 3-4 orders of magnitude larger than in the local ISM or typical giant molecular clouds. It also means that the luminosity of GMCs in ULIRGs must be very high, as they must radiate many times their binding energy over their lifetimes. I briefly review the influence which X-ray irradiation can have on the ISM in AGN-powered ULIRGs. Finally, I show that the presence of PAH features in ULIRGs does not imply that they must be starburst-dominated, since at the column densities and pressures typical of the ISM in ULIRGs PAHs can survive even at tens of parsec distances from the AGN.
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