The relation between the morphological form of a pronoun and its semantic function is not always transparent, and syncretism abounds in natural languages. In a language like English, for instance, three types of indefinite pronouns can be identified, often grouped in series: the some-series, the any-series, and the no-series. However, this does not mean that there are also three semantic functions for indefinite pronouns. Haspelmath (1997), in fact distinguishes nine functions. Closer inspection shows that these nine functions must be reduced to four main functions of indefinites, each with a number of subfunctions: (i) Negative Polarity Items; (ii) Free-Choice Items; (iii) negative indefinites; and (iv) positive or existential indefinites. These functions and subfunctions can be morphologically realized differently across languages, but don’t have to. In English, functions (i) and (ii), unlike (iii) and (iv), may morphologically group together, both expressed by the any-series. Where morphological correspondences between the kinds of functions that indefinites may express call for a classification, such classifications turn out to be semantically well motivated too. Similar observations can be made for definite pronouns, where it turns out that various functions, such as the first person inclusive/exclusive distinction or dual number, are sometimes, but not always morphologically distinguished, showing that these may be subfunctions of higher, more general functions. The question as to how to demarcate the landscape of indefinite and definite pronouns thus does not depend on semantic differences alone: Morphological differences are at least as much telling. The interplay between morphological and semantic properties can provide serious answers to how to define indefinites and the various forms and functions that these may take on.