In this chapter, the taxonomic position of the genus Dunaliella within the Chlorophyta is reviewed briefly, and some of the taxonomic difficulties arising from the use of morphological features at species level, evidenced by molecular studies, are analyzed. The genus Dunaliella comprises 28 recognized species separated in 2 subgenera (Pascheria and Dunaliella) with the subgenus Dunaliella being divided into 4 sections: Tertiolectae, Dunaliella, Virids and Peirceinae. All species belonging to the subgenus Pascheria were found in freshwater and there exist some cytological and molecular arguments suggesting that their taxa do not belong to the genus Dunaliella at all. Within the subgenus Dunaliella, sections are characterized by specific biochemical and physiological attributes while the species have been defined primarily by the morphological criteria. Consequently, the taxonomic validity of some of the 23 morphospecies traditionally described can be questioned. Further, the enormous intraspecific physiological and molecular variability of the hypersaline species Dunaliella salina Teodoresco is highlighted. Finally, some aspects on the major evolutionary trends and phylogeny within the subgenus Dunaliella are discussed.