This Special Issue of Origins of Life and Evolution of Biospheres contains papers based on the contributions presented at the Conference "Defining Life" held in Paris (France) on 4-5 February, 2008. The main objective of this Conference was to confront speakers from several disciplines--chemists, biochemists, biologists, exo/astrobiologists, computer scientists, philosophers and historians of science--on the topic of the definition of life. Different viewpoints of the problem approached from different perspectives have been expounded and, as a result, common grounds as well as remaining diverging arguments have been identified. In addition to individual talks, two large roundtables gave ample room for speakers to discuss their diverging viewpoints. This volume collects almost all the contributions presented during the Conference and provides a rich spectrum of renewed answers to the ever-standing question "What is Life?". Besides the arguments directly regarding this question, more philosophical or historical reflections are also proposed in this issue that were not presented during the Conference. This volume also offers a synthesis written by J. Gayon taking each contribution into account. To conclude this foreword, we would like to thank all the participants and speakers who made this Conference a most stimulating event. Each provided novel ideas to "Defining Life" while highlighting the extreme difficulty to reach a consensus on this topic. We are also very grateful to the French CNRS Interdisciplinary Program Origines des Planètes et de la Vie (Origins of Planets and Life) for its generous support, as well as to the National Museum of Natural History in Paris for hosting the Conference. We also thank Alan W. Schwartz for generously offering this space for publishing the Proceedings of the Conference.