This monograph sets out (i) to establish criteria for differentiating adjectives from other word-classes for languages in which they form a distinct category, and (ii) to establish criteria for determining their (non-)identity with words from other categories for languages in which they do not. As languages show various gradations in the extent to which adjectives can be distinguished from other word-classes, the author discusses idealized language types, thereby providing a model for the analysis of natural languages.