Abstract  Quintessence theories for cosmic acceleration imbue dark energy with a non-trivial dynamics that offers hope in distinguishing the physical origin of this component. We review quintessence models with an emphasis on the 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.