Of the five kingdoms of organisms, the kingdom protista is the most diverse. Kingdom Protista is a link between the Prokaryotic kingdom, Monera and the complex, Eukaryotic, multi-cellular kingdoms fungi, plantae and animalia.

Before the introduction of the five-kingdom system of classification by Whittaker in 1969, the green photosynthetic protophyta were considered as plants; and those protists that ingest their food like animals such as protozoa were considered very small and simple animals. Some protists are decomposer fungi - like protists such as slime moulds.