This book provides an in-depth discussion of the expression of negation in Welsh. It will be the first book length discussion of a single aspect of Welsh syntax since Awbery's (1976) book on Welsh passives. The two main aims of this book are to provide precise and detailed descriptions of the facts and to develop a formal analysis of the main phenomena and consider their implications for theories of syntax. Negation has been a major concern in syntactic theory since the late 1980s. It has been particularly prominent within Principles and Parameters theory but has also had considerable attention within Head-driven Phrase Structure Grammar (HPSG). There has been extensive discussion of negation in English and French, and other Germanic and Romance languages. However, there has been little work on negation in the Celtic languages and very little work on negation in Welsh.