A biological classification is a hierarchical arrangement of species, subspecific units and higher taxa, with the corresponding scientific nomenclature; classification is also the part of systematic biology concerned with generating such an arrangement. Scientific classifications have ancient roots in folk taxonomies. Between the classical Antiquity and the Renaissance, major conceptual advancement were due to Aristotle and Cesalpino, but modern classifications owe mainly to John Ray and eventually to Linnaeus, who introduced binominal nomenclature. Modern classifications are increasingly aiming to mirror phylogenetic relationships, an effort that may eventually require abandoning the traditional Linnaean ranks such as the genus, the family, the order and the class. Nomenclature is ruled by international codes – these provide rules for introducing new names and for selecting the names to be used in the case of conflict between synonymous or homonymous names.

Key Concepts:

• Main steps on the way from naif folk taxonomies to the modern scientific classification initiated by Linnaeus were provided by Aristotle, Andrea Cesalpino and John Ray.
  
• Linnaeus introduced the binomial nomenclature still in use in zoology and botany for the scientific names of species.
  
• Georges Buffon defined the species as a reproductive community, members of which can freely interbreed, thus generating fertile offspring, whereas members of different species, even if similar, cannot breed with them.
  
• For a classification, the equation natural=evolution-based was clearly reinforced by Charles Darwin's (1809–1882) work, especially by his On the Origin of Species (1859). However, Darwin also introduced into biological systematics a potentially disruptive thought: that the species, the hitherto undisputed units of biological classifications, are subject to steady change. In Darwin's own words, species differ from varieties only by matter of degree.
  
• With Charles Darwin, evolutionary biology provided the foundation for a natural classification mirroring genealogical relationships, a research programme actually launched by Willi Hennig one century later.
  
• The last few decades of the twentieth century witnessed heated debates on the theoretical foundations and methodological aspects of biological systematics among the phenetic, cladistics and evolutionary schools.
  
• The traditional Linnaean classification of living beings has the structure of a hierarchy, with a series of ranks or categories (species, genus, family, order, class, phylum and kingdom).
  
• The Linnaean hierarchy has been criticised as its use takes for granted a branched topology and requires acknowledging absolute ranks, rather than simple relations of inclusive nesting.
  
• Biological nomenclature is governed by international codes whose rules are intended to provide unique and universally accepted names for any recognised taxon (species, infraspecific entity or supraspecific group).
  
• Conflicts between synonyms (different names for the same taxon) or homonyms (same name for different taxa) are basically resolved by application of a principle of priority.