Traditionally the green algae were classified in orders or classes according to the morphological species concept. For example, monadoid species (flagellates) were summarized in the order Volvocales, coccoids in the Chlorococcales, filaments in the Ulotrichales or Chaetophorales, and siphonocladous algae in the Cladophorales or Siphonocladales. Later, a new classification was proposed based on ultrastructural investigations of the basal bodies in the flagellar apparatus and cell division. The species with basal bodies in clockwise (CW) or directly opposite (DO) orientation were classified in the class Chlorophyceae, the counterclockwise (CCW)-orientated species in the Ulvophyceae and Trebouxiophyceae (= Pleurastrophyceae). Phylogenetic analyses of nuclear- encoded SSU and ITS rDNA sequences have basically confirmed the classification based on ultra- structural characters. However, most genera and orders are polyphyletic and the relationships between many of the phylogenetic lineages remain unclear. Traditionally taxonomic approaches often depend on single or even negetative “characters” (e.g. absence of zoospore formation or pyrenoids). The authors feel that in some cases these may be given excessive “weight” and advocate the usage of polyphasic approaches (e.g. secondary structures of SSU and ITS rDNA sequences, results of crossing experiments, sporangium autolysin data, and studies of life cycles, multigene approaches, amplified fragment length polymorphism [AFLP]) for the classification of green algae. New generic and species concepts (Z- and CBC-clade concepts, biological species concept, phylogenetic concepts) can be designed for many orders and most of the classes in the Viridiplantae on the basis of this approach.