Are we alone ? Or does life also exist elsewhere in the universe ? This textbook provides an introduction to the exciting new science of astrobiology, which deal with the origins of life on Earth and the possible evolution of life elsewhere in the Universe. The book begins with a detailed examination of how life may have arisen on Earth and introduces the concept of habitable zones. It looks at fossil evidence of life on the early Earth and considers some of the present-day extreme environments that provide analogous habitats for life elsewhere. The evidence for possible life on Mars is reviewed in detail, including a discussion of the Martian meteorite ALH 84001 that has provoked so much debate. The potential for life on the moons Europa and Titan is also determinated. This text also reflects on the possibility of life beyond our solar system. It describes the techniques used to detect exoplanets and reviews our current knowledge of the nature and composition of exoplanetary systems. The book concludes with a look at the scientific basis behind the search for extraterrestrial intelligence.