The Chlorophyceae are one of the classes of green algae, distinguished mainly on the basis of ultrastructural morphology.

The chloroplast may be discoid, plate-like, reticulate, cup-shaped, spiral- or ribbon-shaped in different species.

Depending on the species, Chlorophyceae can grow unicellular (e.g. Chlamydomonas), colonial (e.g. Volvox), filamentous (e.g. Ulothrix), or multicellular.

They share many similarities with the higher plants, including the presence of asymmetrical flagellated cells, the breakdown of the nuclear envelope at mitosis, and the presence of phytochromes, flavonoids, and the chemical precursors to the cuticle.

The presence of sulphur in the culture medium is considered essential for cell division.

It takes place even in the dark with sulphur alone as the source material but under light conditions nitrogen also required in addition.

These observations have an important bearing on the concept of the life cycle of Chlorella, which at present is considered to be strictly asexual in character.

As of May 2023[update], AlgaeBase accepted the following orders in the class Chlorophyceae: Along with these genera, AlgaeBase recognizes several taxa that are incertae sedis (i.e. unplaced to an order): Other orders that have been recognized include: In older classifications, the term Chlorophyceae is sometimes used to apply to all the green algae except the Charales, and the internal division is considerably different.