This course provides a broad introduction to the science of astrobiology, addressing the scientific effort aimed to answer the fundamental question of whether we are alone in the Universe. Specific topics include: structures of different scales in the Universe, formation of stars and their planetary system, origin and condition for life on Earth, other potential sites for life in our solar system, detection of exoplanets in other star systems, and techniques to look life on exoplanets as well as extraterrestrial intelligence.