In December 2014 scientists announced that NASA's Curiosity rover had detected spikes of methane in the atmosphere of Mars and evidence of carbon-based organic molecules in Martian rock samples. Oceans covered much of Mars in the ancient past, and seasonal dark streaks observed on the Red Planet's surface today may be caused by salty flowing water. Oceanic icy worlds have also been found in Jovian moons, Europa, Callisto, and Ganymede; in Saturn's moons, Enceladus and Titan; and probably in Neptune's moon, Triton. Since life as we know it needs water, these revelations are some of the latest in a growing body of evidence leading ever closer to the answer to humanity's greatest question-does life exist beyond planet Earth? Earth revolves around one of billions of stars in the Milky Way, which is but one of billions of galaxies in the known universe. With odds like this, it seems likely that humankind is not alone. The cutting edge of science promises to provide clues to a definitive answer, and steady progress is being made by some of the world's brightest minds. Are We Alone? is a book for the curious that explores complex concepts in an accessible manner. From single-celled organisms to the superintelligent beings of science fiction lore, Are We Alone? examines the intriguing forms that life could take and the conditions required for it to exist beyond earthly confines. This is the story of humankind's greatest philosophical wonder as it is only just beginning to unfurl.