The Einstein Telescope (ET) is a proposed European ground-based gravitational-wave observatory to explore the universe with gravitational waves up to cosmological distances. It is an evolution of the present second-generation detectors such as Advanced LIGO, Advanced Virgo, and KAGRA, leading to a sophis-ticated design including optimum site selection, and could be operating in the mid 2030s. There are several spectacular goals, shortly reported here, that can only be achieved through the detection of gravitational waves with a third-generation detector like ET, and other planned detectors like Cosmic Explorer in the U.S. For other goals, gravitational wave detectors are complementary to facilities exploiting electromagnetic radiation or other messengers, such as neutrinos and cosmic rays. Combined observations through GWs, electromagnetic signals, neutrinos and/or cosmic rays, will give us a multi-messenger and more comprehensive picture of many energetic phenomena of the Universe. The main scientific objectives and the potential for discoveries of ET in astrophysics, cosmology and fundamental physics are briefly reviewed.