Ever since early humans first looked skyward and imagined the stars as distant campfires, humanity has wondered if we are alone in the Universe. The ancient Greeks argued against our home planet being the only cradle for life, but lacked the technology to prove their beliefs. In the late 20th century, the near-simultaneous discoveries of the possible remains of bacterial life in a Martian meteorite, and the first planets orbiting other stars, brought the question of the existence of life beyond the Earth to the forefront of scientific endeavor. In the 21st century, the new field of Astrobiology harnesses the required technological and scientific capability to seriously address this ancient and fundamental question.