One of the characteristic features of Celtic languages is the absence of a singular verbal form with the meaning 'to have'. The principal way of expressing possession is through phrases with prepositions (especially Irish 'ag', Scottish Gaelic 'ag' (at), Welsh 'gan', Breton 'gant' (at; with)). Pronominal prepositions (also known as conjugated prepositions or prepositional pronouns), another distinctive feature of Celtic languages, consist of a preposition and a suffixed pronoun, or rather a pronominal personal ending. Thus the Irish and Welsh equivalents of the English sentence 'I have money' are 'Ta airgead agam' or 'Mae arian gen i', respectively, both literally meaning 'is money at/with me'. The paper discusses the properties of pronominal prepositions in modern Celtic languages and comments on some historical developments leading to this formation.