Determiners are a nominal syntactic category distinct from both adjectives and nouns; they constitute a functional (aka closed or ‘minor’) category and they are typically located high inside the nominal phrasal structure. From a syntactic point of view, the category of determiners is commonly understood to comprise the word classes of article, demonstrative, and quantifier, as well as non-adjectival possessives and some nominal agreement markers. From a semantic point of view, determiners are assumed to function as quantifiers, especially within research informed by Generalized Quantifier Theory. However, this is a one-way entailment: although determiners in natural language are quantificational, their class contains only a subset of the logically possible quantifiers; this class is restricted by conservativity and other factors. The tension between the ‘syntactic’ and the ‘semantic’ perspective on determiners results to a degree of terminological confusion: it is not always clear which lexical items the Determiner category includes or what the function of determiners is; moreover, there exists a tendency among syntacticians to view ‘Determiner’ as naming not a class, but a fixed position within a nominal phrasal template. The study of determiners rose to prominence within grammatical theory during the ’80s both due to advances in semantic theorizing, primarily Generalized Quantifier Theory, and due to the generalization of the X' phrasal schema to functional (minor) categories. Some issues in the nature and function of determiners that have been addressed in theoretical and typological work with considerable success include the categorial status of determiners, their (non-)universality, their structural position and feature makeup, their role in argumenthood and their interaction with nominal predicates, and their relation to pronouns. Expectedly, issues in (in)definiteness, quantification, and specificity also figure prominently in research work on determiners.