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To date, there is nothing that could allow us to figure out if we will ever know whether the Earth is the only inhabited planet in the universe. However, astrobiology has gone a long way since the very first experiments by Miller in 1950 and since then, questions related to astrobiology are feeding the exploration of our solar system, and even beyond with ambitious programs for the observation of extrasolar planets (TPF, Darwin). The answer is maybe already at hand, by a rock analyzed with a Martian rover, or further away, under a few kilometers of ice in the Jupiter neighborhood. . . This chapter deals with the preliminary steps of the astrobiological exploration of the solar system, we are currently witnessing. We focused on the search of organic matter and liquid water, but none of them is a clear indicator of past or present life. We did not discuss the next issue about measurement(s) (chemical, geological, spectroscopic), which could allow us to claim that we have finally detected life on another planet. The scientific debate about it is still at its very first stages, and opens the most fascinating prospects.