  Quintessence theories for cosmic acceleration imbue dark energy with a
non-trivial dynamics that offers hope in distinguishing the physical origin of
the component. We review quintessence models with an emphasis on this dynamics
and discuss classifications of the different physical behaviors. The pros and
cons of various parameterizations are examined as well as the extension from
scalar fields to other modifications of the Friedmann expansion equation. New
results on the ability of cosmological data to distinguish among and between
thawing and freezing fields are presented.
