Case is a grammatical category whose value reflects the grammatical function performed by a noun or pronoun in a phrase, clause, or sentence. In some languages, nouns, pronouns and their modifiers take different inflected forms depending on what case they are in. English has largely lost its case system, although case distinctions can still be seen with the personal pronouns: forms such as I, he and we are used in the role of subject ("I kicked the ball"), while forms such as me, him and us are used in the role of object ("John kicked me").

Languages such as Ancient Greek, Latin, Sanskrit, Russian and Finnish have extensive case systems, with nouns, pronouns, adjectives and determiners all inflecting (usually by means of different suffixes) to indicate their case. A language may have a number of different cases (Latin and Russian each have at least six; Finnish has 15). Commonly encountered cases include nominative, accusative, dative and genitive. A role that one of these languages marks by case will often be marked in English using a preposition. For example, the English prepositional phrase with (his) foot (as in "John kicked the ball with his foot") might be rendered in Russian using a single noun in the instrumental case, or in Ancient Greek as τῷ ποδί tōi podi, meaning "the foot" with both words (the definite article, and the noun πούς pous, "foot") changing to dative form.

As a language evolves, cases can merge (for instance in Ancient Greek genitive and ablative have merged as genitive), a phenomenon formally called syncretism.

More formally, case has been defined as "a system of marking dependent nouns for the type of relationship they bear to their heads. " Cases should be distinguished from thematic roles such as agent and patient. They are often closely related, and in languages such as Latin several thematic roles have an associated case, but cases are a morphological notion, while thematic roles are a semantic one. Languages having cases often exhibit free word order, since thematic roles are not required to be marked by position in the sentence.