The taxonomic hierarchy was formalised by Carl Linnaeus in his landmark works Systema Naturae (1735) and Species Plantarum (1753), establishing the binomial nomenclature system still used today. Linnaeus organised living things into nested, hierarchical groups based on shared structural characteristics, creating a system where each organism has a unique two-part scientific name (genus + specific epithet) and belongs to progressively more inclusive groupings from species up to kingdom. The main taxonomic ranks in the Linnaean system, from most inclusive to most specific, are: Kingdom (e.g., Plantae, Animalia, Fungi), Phylum or Division (e.g., Chordata, Angiospermae), Class (e.g., Mammalia, Dicotyledonae), Order (e.g., Primates, Sapindales), Family (e.g., Hominidae, Anacardiaceae), Genus (e.g., Homo, Mangifera), Species (e.g., sapiens, indica). Additional intermediate ranks (subkingdom, superorder, subfamily, etc.) can be inserted when needed, but the seven main ranks listed above form the core framework used in most biological classification.

The species concept — what exactly constitutes a species — is one of the most fundamental and extensively debated concepts in biology. The Biological Species Concept (BSC), proposed by Ernst Mayr, defines a species as a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. This definition emphasises reproductive compatibility as the key criterion: members of the same species can successfully interbreed and produce fertile offspring; members of different species cannot (due to pre-zygotic or post-zygotic reproductive barriers). While the BSC works well for sexually reproducing organisms with overlapping ranges, it has limitations: it cannot apply to asexual organisms (bacteria, asexual protists), cannot be applied to fossils, becomes ambiguous for organisms in geographic isolation (allopatric populations), and is difficult to apply to plants where interspecific hybridisation and polyploidy are common. Alternative species concepts include the Morphological Species Concept (based on physical form), the Phylogenetic Species Concept (based on shared derived characters and monophyly), and the Ecological Species Concept (based on occupying a distinct ecological niche).

Linnaeus introduced binomial nomenclature — the system of giving each species a unique two-part Latin or Latinised scientific name — which has become the universal standard for biological nomenclature, governed by international codes (International Code of Zoological Nomenclature for animals, International Code of Nomenclature for algae, fungi, and plants). Rules for writing binomial names: The genus name is always capitalised (first letter uppercase), while the specific epithet is never capitalised (all lowercase). Both names are always written in italics or underlined when handwritten. Example: Homo sapiens (humans), Mangifera indica (mango), Panthera leo (lion). The full name can include the authority (the name of the person who first formally described/named the species) and the year, abbreviated as: Homo sapiens Linnaeus, 1758. When the genus name is clear from context, it can be abbreviated to its first letter: H. sapiens, M. indica.

Understanding the taxonomic hierarchy is best achieved through concrete examples. Human classification: Kingdom Animalia → Phylum Chordata → Class Mammalia → Order Primates → Family Hominidae → Genus Homo → Species Homo sapiens. Mango classification: Kingdom Plantae → Division Angiospermae → Class Dicotyledonae → Order Sapindales → Family Anacardiaceae → Genus Mangifera → Species Mangifera indica. Domestic cat: Kingdom Animalia → Phylum Chordata → Class Mammalia → Order Carnivora → Family Felidae → Genus Felis → Species Felis catus. Wheat: Kingdom Plantae → Division Angiospermae → Class Monocotyledonae → Order Poales → Family Poaceae (Gramineae) → Genus Triticum → Species Triticum aestivum. These examples illustrate how the hierarchical classification system places organisms at progressively more specific levels, with closely related organisms sharing more levels of their classification.