We propose to use the low spectral resolution capabilities of IRS to obtain high signal-to-noise mid-infrared spectra of a sample of 70 Ultra- and Hyperluminous extragalactic objects at high redshift discovered in the ELAIS survey. The European Large Area ISO Survey (ELAIS) offers the largest complete mid-IR, high-z, high-L extragalactic sample prior to SPITZER. At any redshift ELAIS provides the most luminous objects easily observable with IRS, being complete to about S (15 um) = 1 mJy, well-matched to the sensitivity of IRS. For this proposal we have selected all those galaxies and AGN which have redshift (determined either spectroscopically or photometrically) larger than one. The sample includes objects confirmed to be type-1 quasars with optical spectroscopy as well as galaxies, fainter in the optical, which are probably dusty luminous starbursts. From the IRS observations we plan to investigate in detail: a) the source of the high bolometric luminosities of the most luminous objects in the distant Universe and compare them with nearby galaxies of similar luminosities, b) the physics of dust and gas in high-redshift objects using well-established mid-infrared spectral diagnostic tools, c) the role of the high-luminosity objects in current models of galaxy formation, and d) the AGN-starburst connection.