PAY ATTENTION: THIS WEEK THE SEMINAR WILL BE ON TUESDAY Hyper-Luminous Infrared Galaxies (HLIRGs) are the most luminous persistent objects in the Universe. The origin of its extreme IR emission is still in discussion, but the combination of AGN and/or starburst (SB) models is generally accepted. We have done a systematic study of a large sample of these objects, combining data in several energy ranges (X-ray, MIR, ...). We have characterized its properties and estimated the relative contribution of AGN and SB components to the bolometric luminosity, finding that both phenomena (accretion to supermassive black holes and intense star formation) are important to explain its whole emission. HLIRGs seem to be unique laboratories to investigate ultimate star formation and its connection to super-massive black hole growth.