Classifiers are partly grammaticalized systems of classification of nominal referents. The choice of a classifier can be based on such criteria as animacy, sex, material, and function as well as physical properties such as shape, size, and consistency. Such meanings are expressed by free or bound morphemes in a variety of morphosyntactic contexts, on the basis of which particular subtypes of classifiers are distinguished. These include the most well-known numeral classifiers which occur with numerals or quantifiers, as in Mandarin Chinese yí liàng chē (one clf.vehicle car) ‘one car’. The other types of classifiers are found in contexts other than quantification (noun classifiers), in possessive constructions (possessive classifiers), in verbs (verbal classifiers), as well as with deictics (deictic classifiers) and in locative phrases (locative classifiers). Classifiers are found in languages of diverse typological profiles, ranging from the analytic languages of Southeast Asia and Oceania to the polysynthetic languages of the Americas. Classifiers are also found in other modalities (i.e., sign languages and writing systems). Along with grammatical gender, classifiers constitute one of the two main types of nominal classification. Although classifiers and gender differ in some ways, with the presence of a classifier not being reflected in agreement (i.e., the form of associated words), in others they exhibit common patterns. Thus, both types of nominal classification markers contribute to the expansion of the lexicon and the organization of discourse. Shared patterns also involve common paths of evolution, as illustrated by the grammaticalization of classifier systems into gender systems. In turn, particular types of classifiers resemble various means of lexical categorization found in non-classifier languages, including measure words, class terms, as well as semantic agreement between the verb and direct object. All these three means of classification can be viewed in terms of a continuum of grammaticalization, ranging from lexical means to partly grammaticalized classifiers and to grammaticalized gender systems. Although evidence of classifiers in non-Indo-European languages has been available since the 16th century, it was only the end of the 20th century that saw a formative stage in their study. Since then, classifier systems have offered fascinating insights into the diversity of language structure, including such key phenomena as categorization, functionality, grammaticalization, and the distinction between lexicon and grammar as well as the language-internal and external factors underlying the evolution of morphosyntactic complexity.