双鞭毛虫是一组有鞭毛的单细胞生物，被认为是藻类的一种。 它们的名字来自希腊语dinos，意思是 "旋涡"，鞭毛的意思是鞭子。 这是指它们在水中移动时，使用两根鞭毛的特点：一根纵向鞭毛和一根横向鞭毛。 双鞭毛虫构成了真核生物（复杂单细胞）海洋浮游生物的主要部分，数量仅次于硅藻。

Dinoflagellates are known as the source of red tides and one of the sources of oceanic bioluminescence. "Red tides," also known as harmful algal blooms, occur when phytoplankton (photosynthesizing protists) release harmful chemicals into the water to kill fish and other organisms that threaten to eat them. These chemicals can get into airborne moisture and irritate the lungs and mucus membranes of people on the beach. The phenomenon is most common on the Gulf Coast of Florida. Sometimes red tides are cased by human activity, such as agricultural runoff, which provides an abundant nutrient source to the dinoflagellates. Harmful algal blooms are also associated with oxygen depletion, as the organisms in the red tide take up a lot of oxygen and then perish, the oxygenated molecules in their bodies sinking to the bottom of the sea.

Dinoflagellates have characteristics of both plants and animals. They can photosynthesize, but they are also mobile. Dinoflagellates have various adaptations to survive microscopic predators. Some species have cellulose armor called a theca, while others have body spikes called extrusomes, which can also release substances such as harmful chemicals outside the cell. Dinoflagellates reproduce both asexually, by dividing, and sexually, by combining with another member of its species and forming a zygote. The zygote releases substances that cause it to be enclosed within a cyst, in a process called encystment. After a certain period of time, the cyst breaks, and the cell divides, fresh with new genetic material. The cysts, called dinocysts, are the only representatives of dinoflagellates in the fossil record.

甲藻类可以被概括为小（尽管有些物种大到2毫米）、浮游生物（90%是海洋浮游生物）、可移动，并且通常有盔甲。 一些甲藻类实际上没有光合作用，但大多数是。 甲藻类的特点还在于其紧凑的基因组，其中一些已经被完全测序。