Ultraluminous Infrared Galaxies and the Origin of QSOs

We review the evidence which suggests that ultraluminous infrared galaxies (ULIGs) are the precursors of optically selected quasi-stellar objects (QSOs) and discuss additional data that suggests that the majority, if not all QSOs, may begin their lives in an intense infrared phase. Implications for the host galaxies of QSOs are discussed.