Résumé
The observed recent accelerated expansion of the Universe constitutes a dramatic departure from traditional views. The mechanism behind this accelerated expansion is still mysterious and is a major challenge to present-day cosmology. Though it can be due to a new perfect fluid with negative pressure coined Dark Energy, it could also originate from modifications of gravity. We present some Dark Energy models in the context of General Relativity and review in some details the properties of Dark Energy models in scalar-tensor gravity. We emphasize that observations probing both the background and the perturbations will constrain the proposed model most effectively.