R.H. Whittaker, an American ecologist proposed a five kingdom system of classification in 1969. The kingdoms defined by him were named Monera, Protista, Fungi, Plantae and Animalia. The main criteria for classification used by him include cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships.

Basic features of Five Kingdom Classification:
1. Structure of Cells: Organisms can be divided into two major categories on the basis of cell structure:

1. Prokaryotic cells: having incipient nucleus and
2. Eukaryotic cells: having membrane bounded nucleus and cell organelles.

2. Body Organisation: The theory of organic evolution has revealed that the earliest organisms were simple unicellular. Some of the unicellular prokaryotic organisms in due course of time evolved into multicellular organisms. Some of the multicellular organisms finally became complex.

3. Mode of Nutrition: R.H. Whittakar classified organisms on the basis of autotrophic and heterotrophic mode of nutrition. Autotrophic mode of nutrition can be divided into chemosynthetic nutrition and photosynthetic nutrition. Heterotrophic organisms are also divided into two categories: holozoic or ingestive and saprotrophic or absorptive.

4. Phylogenetic Relationships: Evolutionary history of an organism is called phylogeny. Therefore, phylogenetic relationships of organisms alone are the index of their true kinship. Such relationships are not clear in all cases. Only fossil record can provide such relationships but this record is incomplete and is likely to remain so. Therefore, the phylogenetic relationships are ascertained from morphology, cytology, genetics, biochemistry and physiology for the correct classification of living organisms.