[3] Members of this class are single-celled, colonial, or multicellular and are found in freshwater, terrestrial or marine habitats worldwide.

[4] A number of taxa have also lost the ability to photosynthesize, and have evolved to become parasitic; examples include Prototheca and Helicosporidium.

[9] Chloroplasts are diverse in morphology and placement and may be axial and stellate (e.g. Prasiola), numerous and discoid (e.g. Eremosphaera), or parietal.

It has basal bodies of its flagella in a counterclockwise orientation, non-persistent, metacentric spindles during telophase, and phycoplast-mediated cytokinesis.

Because the morphology of these algae is often heavily influenced by the environment, reliable identification relies on molecular techniques such as DNA barcoding.

[11] In addition to lichens, single-celled microalgae (called zoochlorellae) are commonly found as endosymbionts within a variety of freshwater and marine organisms.

[17] The clade Trebouxiophyceae likely originated about 600–800 million years ago,[10] and fossils from trebouxiophycean algae have appeared in sediments dating back to at least the Precambrian.

[18] The closest relatives of Trebouxiophyceae are Chlorophyceae and Ulvophyceae; together, they form a monophyletic group termed the UTC clade (also known as core Chlorophyta).

It is hypothesized that the production of autospores became advantageous in terrestrial environments, since flagellated cells require water for movement.

[21] As of 2025, AlgaeBase accepts the following orders:[3] Genera and families without intervening taxonomy include:[3] The microalga Chlorella has been used by researchers to study basic elements of biochemistry and physiology, as an simpler analog of land plants.

[27] Since then, improvements in crop yield have caused a decline in interest in Chlorella as food;[27] however, it has a small market as a niche nutritional supplement.