Einstein Telescope (ET) will be an Observatory of Gravitational Waves (GW) of future generation. It builds on the success of current, second-generation detectors Advanced Virgo and Advanced LIGO, whose breakthrough discoveries of merging black holes (BHs) and neutron stars over the past decade have ushered scientists into the new era of GW astronomy. ET consists of a set of multi-km arm optical interferometers with suspended optics. Each ET detector is composed by two interferometers: ET-LF devoted to the detection of the GW in the Hertz frequency regime, and ET-HF aiming to detect the GW signal up to few kHz. ET will achieve a greatly improved sensitivity by increasing the size of the interferometer from the 3km arm length of the Virgo detector to 10-15km, and by implementing a series of new technologies. These include a cryogenic system to cool some of the main optics to 10 – 20K, new quantum technologies to reduce the fluctuations of the light, and a set of infrastructural and active noise-mitigation measures to reduce environmental perturbations. The expected improvement of the sensitivity with respect to the current detectors is about one order of magnitude at high frequency and several orders at low frequency. ET will make it possible, for the first time, to explore the Universe through GW along its cosmic history up to the cosmological dark ages, shedding light on open questions of fundamental physics and cosmology. It will probe the physics near black-hole horizons, help understanding the nature of dark matter, and the nature of dark energy and possible modifications of general relativity at cosmological scales. Exploiting the ET sensitivity and frequency band, the entire population of stellar and intermediate mass black holes will be accessible over the entire history of the Universe, enabling to understand their origin (stellar versus primordial), evolution, and demography. ET will observe the neutron-star inspiral phase and the onset of tidal effects with high SNR providing an unprecedented insight into the interior structure of neutron stars and probing fundamental properties of matter in a completely unexplored regime (QCD at ultra-high densities and possible exotic states of matter). The excellent sensitivity extending to kHz frequencies will also allow us to probe details of the merger and post-merger phase. ET will operate together with a new innovative generation of electromagnetic observatories covering the band from radio to gamma rays (such as the SKA, the Vera Rubin Observatory, E-ELT, CTA). Two candidate sites, one in Sardinia and one in the Euregio Meuse-Rhine, and two geometries, triangular and 2L shaped, are under investigation. The science potential and the feasibility of a single-site solution, located either in the Sardina candidate site or in the Euregio Meuse-Rhine site is compared with respect to the possibility to realize a 2L geometry distributed in the two candidate sites.