We review investigations with SWS, ISOPHOT-S, and ISOCAM-CVF of several samples of ultra-luminous infrared galaxies (ULIRGs) drawn from IRAS catalogs. The mid-IR data indicate that the majority of the local ULIRG population is predominantly powered by star formation but that AGN activity plays a signi.cant role in many sources and dominates the most luminous ones. AGN and starburst activity is present in all phases of the ULIRG/merger evolution. There is no obvious trend for AGNs to be more predominant, or the molecular content to be lower in the late ULIRG phases. Star formation activity in ULIRGs likely occurs in short (107 years) phases terminated by the disruptive effects of superwinds and supernovae. Highly obscured AGNs dominating the total energy output may be hidden to the mid-infrared observations in a few cases but are not characteristic of the average ULIRG.