  A large number of recent observational data strongly suggest that we live in
a flat, accelerating Universe composed of $\sim$ 1/3 of matter (baryonic +
dark) and $\sim$ 2/3 of an exotic component with large negative pressure,
usually named {\bf Dark Energy} or {\bf Quintessence}. The basic set of
experiments includes: observations from SNe Ia, CMB anisotropies, large scale
structure, X-ray data from galaxy clusters, age estimates of globular clusters
and old high redshift galaxies (OHRG's). Such results seem to provide the
remaining piece of information connecting the inflationary flatness prediction
($\Omega_{\rm{T}} = 1$) with astronomical observations. Theoretically, they
have also stimulated the current interest for more general models containing an
extra component describing this unknown dark energy, and simultaneously
accounting for the present accelerating stage of the Universe. An overlook in
the literature shows that at least five dark energy candidates have been
proposed in the context of general relativistic models. Since the cosmological
constant and rolling scalar field models have already been extensively
discussed, in this short review we focus our attention to the three remaining
candidates, namely: a decaying vacuum energy density (or ${\bf \Lambda(t)}$
{\bf models}), the {\bf X-matter}, and the so-called {\bf Chaplygin-type gas}.
A summary of their main results is given and some difficulties underlying the
emerging dark energy paradigm are also briefly examined.
