绿藻是一种微生物，可以在包括海水、淡水和汽水等所有的水中环境里被找到。

绿藻是为有胚植物根源的藻类中的一大类群，本身是一个并系群，有时被归在植物界，有时则又被归在原生生物界里。绿藻是单细胞或群集的鞭毛生物，一般一个细胞有两个鞭毛，但也会有群集、粒状和丝状等不同的型式。车轴藻是最接近高等植物的近亲，存在着完全分化的身体组织。绿藻约有6000个物种。许多物种都是单细胞的，但有其他少数形成群集或长条的丝状。

少数其他生物会依靠绿藻行光合作用来得取能量。有人猜想眼虫和chlorarachniophytes的叶绿体是消化绿藻后得来的，而后者还有残留的细胞核（类核体）。某些种类的绿藻，特别是共球藻属或伪共球藻属内的绿藻，和真菌之间可以找到形成地衣的共生关系。一般而言，有共生关系的真菌不能独自存活，但其藻类物种却通常可以在自然界中发现不依靠真菌而存活着。

The green algae (singular: green alga) are a large, informal grouping of algae consisting of the Chlorophyte and Charophyte algae, which are now placed in separate Divisions.

The land plants or Embryophytes (higher plants) are thought to have emerged from the Charophytes. As the embryophytes are not algae, and are therefore excluded, green algae are a paraphyletic group. However, the clade that includes both green algae and embryophytes is monophyletic and is referred to as the clade Viridiplantae and as the kingdom Plantae. The green algae include unicellular and colonial flagellates, most with two flagella per cell, as well as various colonial, coccoid and filamentous forms, and macroscopic, multicellular seaweeds. In the Charales, the closest relatives of higher plants, full cellular differentiation of tissues occurs. There are about 8,000 species of green algae. Many species live most of their lives as single cells, while other species form coenobia (colonies), long filaments, or highly differentiated macroscopic seaweeds.