I propose to continue a wide-ranging study of the most massive stars in the Local Universe, which I have undertaken as the Giacconi Fellow at the Space Telescope Science Institute (Baltimore, USA). The study involves discovering and measuring the parameters of massive stars in eclipsing binaries both in the Milky Way and in nearby galaxies. Determining accurate fundamental parameters for dozens of massive stars at a range of metallicities will for the first time provide significant constraints for massive star formation theories, models of stellar evolution and the elusive upper stellar mass limit. The proposed survey will contribute to the understanding of these rare objects by providing the fundamental stellar properties: the mass and radius, for stars located in a variety of environments. During the past 3 years, I have obtained extensive data and first results in the investigation of massive stars. A Marie Curie Fellowship at the National Observatory of Athens (Greece) will allow me to extend and deepen this effort and to bring it to conclusion, as well as to transfer the expertise acquired over the past 12 years in the USA to the host institution.