The mission “Extreme Universe Space Observatory - EUSO” is devoted to the investigation of the Extreme Energy Cosmic Rays (EECR withE>5×1019&hthinsp;eV) and of the High Energy Cosmic Neutrino flux looking at the streak of UV fluorescence light produced when the particles coming from Outer Space interact with the Earth’s atmosphere. EUSO will observe the fluorescence signal looking downward from Space the dark Earth atmosphere under a 60 degrees full field of view; the Fluorescence light will be imaged by a large Fresnel lens optics into a finely segmented focal plane detector. The segmentation and the time resolution adopted will consent to reconstruct the shower arrival direction and energy with high precision, EUSO is a collaborating effort of 27 research groups from Europe, U.S.A, and Japan and it has been designed to operate for more than 3 years mission life-time; it is expected to detect of the order of103/year EECRs withE>1020&hthinsp;eVand to open a window into the High Energy Neutrino Astronomy. Originally proposed to the European Space Agency as a Free Flyer Low Earth Orbit Mission, EUSO has been approved by ESA in march 2000 for an Accommodation Study on the International Space Station, with a goal for flight in 2006. ©2001 American Institute of Physics.