The "Book of Aneirin" is a thirteenth-century manuscript collection of Welsh praise-poetry. In comparison with other Welsh sources of similar date, the language of this text exhibits a number of features which have been interpreted as archaisms and taken as indications of great antiquity for the text. However, particularly in syntax, claims about the status of these 'archaisms' have not been discussed in the context of the grammatical organisation of the text as a whole. This book approaches various aspects of grammar against the background of a comprehensive edition of the finite verbal clauses of the text. Syntactic analysis of the data-base so established takes its point of departure from the relationship of the verb with its arguments in the clause, and is concentrated on two issues: 1. the type and status of basic word order in the text; 2. the interaction of the semantics of the predication with the pragmatics of communication of information. It is argued that, as would be expected for a Welsh text, the basic order is VSO, but also, and more importantly, that the text does not contain 'archaic' evidence of any earlier, different basic orders. Rather it is argued that word-order variation in the text can be rigorously analysed in terms of a model of functional syntax which is sensitive to both the pragmatics of the text and the semantics of the predications involved. In the light of these results, argumentation concerning historical syntax and especially reconstruction of syntax are evaluated, both in the field of Celtic and in wider cross-language perspective. Finally, the edition of the finite clauses of the text is followed by a number of notes discussing historical and synchronic aspects of the material presented, with particular emphasis on morphology and etymology.