This is a group of about 1500 marine species. Most live in the rocky intertidal or nearby subtidal regions. These algae are especially abundant and conspicuous in colder marine water. Most are attached to rocks and don’t survive for long if they break free. An exception would be Sargassum. Sargassum continues to grow by fragmentation when detached. Large quantities accumulate in an eddy of the Atlantic Ocean called the Saragasso Sea.

Vegetative growth in the simplest forms of brown algae is filamentous. In some species, filaments are intertwined and compacted, leading to false 3-D tissue called "pseudoparenchyma." The range of diversity with true 3-D tissue is impressive. Some reach tree-like proportions and form underwater subtidal forests. (Fig.1) The large array of forms in the brown algae can be accounted for in part by differences in the way that new cells are added. Cells can either be added throughout the plant body (diffuse or generalized growth), or in discrete regions called meristems. Meristems can be apical, basal, somewhere in between (intercalary), or on marginal surfaces. In most of the 3-D growth forms, the thallus differentiates into a photosynthetic blade supported by a stalk (or stipe) and attached to rocks by a basal "holdfast." (Fig.2) In some forms air bladders help keep blades afloat when algae are submerged. Mucilaginous material in cell walls helps keep these algae from drying out during exposure at low tide. Alginate type mucilage in some species is of considerable economic importance. After extraction from harvested plants (Fig.3) it is used as an emulsifier or thickening agent in ice cream, chocolate milk, paint, beer, and many other products. Some brown algae are edible and are commercially packaged for food (Fig.4).

The brown algae produce cells that are eukaryotic. Photosynthetic pigments are chlorophyll a, chlorophyll c, and fucoxanthin. Excess food is stored in the form of a soluble glucose polymer called laminarin. Laminarin is similar chemically to the chrysolaminarin found in diatoms. Flagellated spores and gametes are produced during sexual reproduction in brown algae. As in some diatoms, there are two flagella per cell – one whiplash, one "tinsel." Flagella insertion is always lateral.

Three distinct types of life cycles can be found in the brown algae. In Ectocarpus, for example, haploid filaments produce gametes in gametangia (Fig.5). Diploid filaments produce flagellated meiospores in sporangia (Fig.6). Except for the sexual organs, the haploid and diploid filaments are morphologically similar. In Laminaria, there are also haploid and diploid generations, but the haploid phase is microscopic. In a third type, exemplified by Fucus, there is no multicellular haploid phase. The diploid plants produce gametes by meiosis. Along with growth forms, these life cycle types are used to distinguish between the various orders of brown algae.

The earliest fossils similar in appearance to modern brown algae appear in the late Devonian. Brown algae fossils remain rare until just before the Triassic period. Unlike red and green algae that have calcified walls, the brown algae do not fossilize well. Based on molecular and other data, this group is related to the golden algae (e.g., diatoms) and other heterokonts.