The discovery of cosmic acceleration is one of the most important
developments in modern cosmology. The observation, thirteen years ago, that
type Ia supernovae appear dimmer that they would have been in a decelerating
universe followed by a series of independent observations involving galaxies
and cluster of galaxies as well as the cosmic microwave background, all point
in the same direction: we seem to be living in a flat universe whose expansion
is currently undergoing an acceleration phase. In this paper, we review the
various observational evidences, most of them gathered in the last decade, and
the improvements expected from projects currently collecting data or in
preparation.Comment: Accepted review article to appear in a special volume of the "Comptes
Rendus de l'Acad\'emie des Sciences" about Dark Energy and Dark Matte
