Classification systems for living organisms have undergone several changes over time. While Plantae and Animalia kingdoms have remained constant, the understanding of what groups should be included under these kingdoms has been changing. The number and nature of other kingdoms have also been understood differently by different scientists.

Since the dawn of civilisation, classification was done instinctively based on human needs for food, shelter, and clothing. Aristotle was the earliest to attempt a more scientific basis using simple morphological characters to classify plants into trees, shrubs, and herbs, and animals into those with and without red blood.

During Linnaeus' time, a Two Kingdom system with Plantae and Animalia was developed. However, this system was found inadequate as it did not distinguish between eukaryotes and prokaryotes, unicellular and multicellular organisms, and photosynthetic and non-photosynthetic organisms. Many organisms did not fall into either category.

A need was felt for including other characteristics like cell structure, nature of wall, mode of nutrition, habitat, methods of reproduction, and evolutionary relationships. Classification systems now aim to reflect morphological, physiological and reproductive similarities and are also phylogenetic, i.e., based on evolutionary relationships. Such changes will continue in future with improvement in our understanding.