It has often proven difficult to classify certain words as adpositions or nouns, verbs, adjectives, etc. This book looks at the distinctions between adpositions, i.e. prepositions and postpositions, and other word classes with respect to a wide range of languages. In particular, it focuses on how these distinctions have been treated by previous authors and the terminology used to describe items on or close to the adpositional border, e.g. pseudo-postpositions and auxiliary nouns. Chapters are devoted to adpositions as opposed to most of the other traditional parts of speech. Among the criteria for (non-)adpositional status brought up are the presence or absence of inflection on putative adpositions and genitive case marking on complements of such words. Definitive conclusions on how to determine whether words are adpositions seem elusive, but some formal criteria, such as absence of inflection, are problematic; possibly a solution will involve a notion of adpositional function.