This chapter examines the notion of case, establishing the extent to which such a feature is needed in the grammars of individual languages. Case systems represent an advanced stage in the grammaticalization of lexical concepts, in which lexical meaning is lost and the cases function as exponents or signals of other grammatical categories such as grammatical functions. Although there are distinct allomorphs of case markers, the distribution of these allomorphs is usually determined by phonology. Morphological and syntactic cases are distinct properties and, in principle, can be found independent of each other. In morphology, an attribute of the morphological case is needed to generalize over inflectional classes. In the syntax, there is a need for an attribute of the syntactic case to state certain types of agreements, multiple-case marking, and some forms of government. The inflection/derivation distinction is a metagrammatical distinction that can only be drawn by grammarians by considering the interaction of all the principles of morphology, syntax, semantics, and lexical relatedness in the language.