Green and brown algae are two groups of algae that together make up most of the algae in the world, though they are quite different. Along with red algae, both brown algae and green algae are sometimes referred to using the colloquial term "seaweeds." Though both are eukaryotic (complex-celled) multicellular organisms, they belong to different kingdoms, with green algae belonging to Plantae and brown algae to Chromalveolata. Plantae and Chromalveolata are two of the six major eukaryote divisions, the others being Fungi, Animalia, Amoebozoa, Rhizaria, and Excavata. Both groups of algae are mostly marine, but green algae is better adapted to fresh water than brown algae.

Brown algae is most familiar to us as kelp, marine algae with a very high growth rate, and Sargassum, a surface-floating variety of brown algae that makes up the Sargasso Sea and provides a unique habitat for eels and other animals. Though kelp and Sargassum are the best known brown algae, there are over 1,500 species in total, being especially common as seaweeds in the colder Northern Hemisphere. Brown algae can often be found along rocky shores. Along with their mostly unicellular relatives in the phylum Heterokontophyta, brown algae are autotrophs (photosynthesizing organisms) with chloroplasts covered in four membranes. Brown algae uses a pigment called fucoxanthin to absorb sunlight, giving it a brownish-green color. Cells within a brown algae often have holes which they use to share nutrients and free carbon.

Green algae is somewhat more common than brown algae from the perspective of humans, as it grows more frequently in and near lakes and rivers, which we tend to see more often than the open sea. Green algae is famous for being the most primitive group in the kindgom Plantae, and the form of life from which land plants (embryophytes) evolved approximately 500 million years ago, during the Ordovician period. There are about 6000 known species of green algae, most of it unicellular, though the most visible species live in colonies structured in long chains or filaments. Only in order Charales, the stoneworts, a type of pond weeds most closely related to land plants, does true tissue differentation occur.

Both types of algae are extremely crucial as producers in aquatic ecosystems, and the diet of many fish, especially juveniles, is mostly or exclusively algae. Some fish are even specially adapted to cleaning algae off of other fish. Alongside corals, kelp forests create one of the most species-rich and complex aquatic ecosystems on the planet, home to tens or thousands of marine species.