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Is there water in the universe? Is there water in the solar system elsewhere than on Earth? Water is the 2nd most abundant molecule in the universe, after dihydrogen. It is found everywhere in the cosmos, from the planets of the solar system to distant galaxies, but always in the form of vapor or ice. Liquid water is an essential ingredient for all life on Earth. With 70% of its surface covered by water, the Earth is the only oasis. But where does the water on Earth come from? Was it already there when our planet was formed? It seems that liquid water may have existed on Venus or Mars, but why did it disappear? So many questions that have long fascinated many scientists. By searching for water in the Universe, astronomers are trying to unravel the mysteries of the birth and evolution of planets, as well as the appearance of life on Earth. And with the recent discovery of several thousand extrasolar planets, this fundamental question arises, which is perhaps one of the greatest challenges of science today: are we alone in the universe?