[Systems biology: the multiscale organization of living systems].

Complexity of biological systems originates not only from their emergent properties but also from the feedbacks exerted by these properties on elementary structures and mechanisms. A systemic approach is required to capture this multiscale organization and its among-level feedback loops which have resulted from selective pressure during evolution: the scientific challenge is not to take into account all the relevant details but rather to consider jointly several integration levels. New mathematical tools are necessary to express how the consistency and interrelations among these different levels control biological functions.