Is it possible that extraterrestrial life forms exist within our Galaxy, the Milky Way? This book offers a critical analysis by leading experts in a range of sciences, of the plausibility that other intelligent lifeforms do exist. Exploration of the Solar System, and observations with telescopes that probe deep space, have come up empty handed in searches for evidence of extraterrestrial life. Many experts in the fields of astronomy, biology, chemistry and physics are now arguing that the evidence points to the conclusion that technological civilisations are rare. After ten billion years, and among hundreds of billions of stars, we may well possess the most advanced brains in the Milky Way Galaxy. This second edition contains many new and updated aspects of extraterrestrial research, especially the biological viewpoint of the question.