DARWIN is a space mission concept proposed in the framework of the European Space Agency `Call for Ideas' for the Horizon 2000+ plan. It aims at detecting terrestrial exo- planets by interferometric coronography in the infrared, then searching for the presence of atmospheric ozone which traces the presence of an active biological photosynthesis. The concept itself is elaborated on ideas previously proposed and developed by Bracewell, Owens, Angel et al.,...Its main originality is that we believe that the goal of detecting extra-solar life can be achieved with a reasonably simple instrumental concept, i.e. passively cooled medium- size (approximately 1 m diameter) space telescopes located at more than 3.5 AU from the Sun in order to avoid the bulk of the zodiacal thermal emission.