This book uses modern biological knowledge to tackle the question of what distinguishes living organisms from the non-living world. The authors first draw on recent advances in cell and molecular biology to develop an account of the living state that applies to all organisms (and only to organisms). This account is then used to explore questions about evolution, the origin of life, and the possibility of extraterrestrial life. The novel approach taken by this book to issues in biology will interest and be accessible to both the general reader as well as students and specialists in the field.