Cocconeis placentula is among the most ubiquitous and widely studied freshwater and brackish-water diatoms worldwide, serving as a model organism for periphyton ecology and a key species in biological assessment of water quality. A member of the family Cocconeidaceae, this adnate epiphyte produces a characteristic ovoid frustule with fine transapical striae, a well-developed raphe system on one valve, and an elaborate pattern of areolae visible under electron microscopy. The species complex encompasses several morphological varieties (var. placentula, var. lineata, var. euglypta, var. acuta) that differ in fine structural details of the valve ornamentation. Cocconeis placentula colonizes the surfaces of aquatic plants, algae, rocks, and sediment particles in rivers, lakes, and ponds across a vast global range spanning Europe, the Americas, Asia, Africa, and beyond, tolerating a wide range of water temperatures, pH values, and moderate nutrient enrichment. Its abundance varies dramatically with substrate type, light availability, and water chemistry, making community composition data involving this species valuable in diatom-based paleolimnological reconstructions and modern bioassessment indices. As a benthic primary producer, C. placentula contributes substantially to the productivity of the photic benthos and serves as a dietary component for invertebrate grazers. Its remarkable cosmopolitan distribution and ecological flexibility make it one of the defining species of freshwater periphyton communities globally. Conservation status is not formally assessed.