  There is some evidence that the Universe is presently undergoing accelerating
expansion. This has restored some credit to the scenarios with a non-vanishing
cosmological constant. From the point of view of a theory of fundamental
interactions, one may argue that a dynamical component with negative pressure
is easier to achieve. As an illustration, the quintessence scenario is
described and its shortcomings are discussed in connection with the nagging
``cosmological constant problem''.
