An ultra-luminous infrared galaxy (ULIRG) is a galaxy that emits most of its total energy output in the far (long-wavelength) infrared – a sure sign that it contains a huge amount of dust. Such large dust concentrations, in turn, suggest that ULIRGs are the scene of prodigious star formation. Although ULIRGs are still found in the universe today, recent surveys suggest they were much more common in the remote past and contribute significantly to the cosmic background in the far-infrared and submillimeter bands.