Cosmological constant versus quintessence

The mounting evidence that the universe is presently undergoing acceleratin g expansion has restored some credit to the scenarios with a nonvanishing c osmological constant. From the point of view of a theory of fundamental int eractions, one may argue that a dynamical component with negative pressure is easier to achieve. As an illustration, the quintessence scenario is desc ribed and its shortcomings are discussed in connection with the nagging "co smological constant problem.".