Diatoms are photosynthetic plankton that form large populations in fresh and marine water habitats. They are single-celled or colonial organisms. Cell structures in the group are eukaryotic. Vegetative cells have nuclei containing a diploid number of chromosomes. Photosynthetic pigments are similar to those found in brown algae, i.e., chlorophyll a, chlorophyll b, and fucoxanthin. Excess food is stored either as glucose polymer called chysolaminarin, or as oils.

The walls of diatoms are made of silica. They consist of two overlapping halves, somewhat like a petri dish but much smaller and with more elaborate sculpturing. Shape and sculpturing patterns are species specific. Two basic types of diatoms are distinguishable based on symmetry. Radial symmetric ones are called centric diatoms (Fig.1) whereas bilateral symmetric ones are called pennate diatoms (Fig.2). The two wall halves of each cell in either type are recycled following cell division. Each daughter cell keeps one half. The new half for each cell forms within the old one. A large proportion of cells in a growing population becomes smaller as a result of this process. Very small cell size in diatoms triggers sexual reproduction.

Nuclei of the smallest cells in the diatom population undergo meiosis. The products of meiosis are usually amoeboid-like gametes, though flagellated male sperm are formed in a few marine species. Silica walls of the mother cells are discarded, releasing gametes into open water. Gamete pairing produces a diploid zygote called an auxospore. Auxospores then enlarge to the maximum size of the species before forming new silica walls.

There are at least one hundred thousand living species of diatoms and thousands more extinct ones. Silica is very resistant to decay, so the walls of extinct species make great fossils. Diatoms become abundant and reliably identified in the fossil record about one hundred million years ago during the Cretaceous period. Settling of their walls over geologic time resulted in rich deposits of "diatomaceous earth." Such deposits are an important economic resource, used for example in filtration systems and as abrasives in polish and toothpaste. The group shares many characteristics with golden algae (e.g., the silicoflagellates) and the haptophytes (e.g., the coccolithophorids). Some taxonomists lump all three groups together. Characteristics that ally these groups with the brown algae include photosynthetic pigments, food reserve, and flagellated cell types.