生物可以根据构成的细胞数目分为单细胞生物和多细胞生物。单细胞生物只由单个细胞组成，而且经常会聚集成为细胞集落。单细胞生物能独力完成新陈代谢及繁殖等活动。

地球上最早的生物大约在距今35亿至41亿年前形成，原核生物是最原始的生物，如细菌和蓝绿藻且是在温暖的水中发生。

单细胞生物包括所有古细菌和真细菌和很多原生生物。根据旧的分类法有很多动物，植物和真菌多是单细胞生物。变形虫算作单细胞动物，它的一些种类却算作粘菌，带鞭毛的鞭毛虫如眼虫有时被归为单细胞藻类或者是单细胞动物。新的分类法中，所有的真核单细胞生物都算作原生生物。

A unicellular organism, also known as a single-celled organism, is an organism that consists of only one cell, unlike a multicellular organism that consists of more than one cell. Historically the simple single celled organisms have sometimes been referred to as monads, specially the unicellular flagellates. The main groups of unicellular organisms are bacteria, archaea, protozoa, unicellular algae and unicellular fungi. Unicellular organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. Unicellular organisms are thought to be the oldest form of life, with early protocells possibly emerging 3.8-4 billion years ago.