Taxonomy, the branch of biology that deals with the classification of living and extinct organisms. It is concerned with distinguishing individual organisms, placing them in groups according to kinship, and arranging the groups in an orderly, meaningful, and useful system. Taxonomy includes not only the study of the structure, function, and development of an individual organism but also the study of the evolutionary development of the group to which it belongs.

Scientific classification is the method scientists have developed to arrange all of the world's organisms both extinct and living things into groups. They base their classification system on the biological similarities that exist among species (kinds) of organisms. Classification creates a method for organizing facts about organisms and groups of organisms.

Scientists keep making changes to classification. Since the mid-1900’s, biologists have gathered a lot of information on classification from studying the sequence (order) of compounds called bases within genes in the genomes of different species. A gene is the part of a cell that determines which traits an organism inherits from its parents. The term genome refers to all the genes on a cell’s chromosomes, structures that consist mainly of the genetic material DNA (deoxyribonucleic acid). By comparing genomes of different species, biologists have more accurately determined how these species are related to one another.

Scientists also use a method of analysis called cladistics in which they evaluate anatomical traits of at least three related species. These evaluations help them construct a cladogram, a kind of family tree illustrating how the species are related to one another.