The present Etymological Glossary of Old Welsh is intended to offer an alphabetically arranged list of words which are found in the manuscripts transcribed before the beginning of the Middle Welsh period, and to provide them with the most important published references. Only the records written down during the Old Welsh period have been used is the compilation of the glossary. The only text which was not used is the Book of Llan Dav, which still requires to be comprehensively discussed, and is a subject for research on its own right. The data of this very important document is used throughout as comparanda for the research. The focus has been laid on the collection of the published analysis of the rudiments of Old Welsh; thus the glossary could be viewed as an extended bibliography for Old Welsh studies. The entries are arranged alphabetically according to the Welsh standard. The glosses which contain more than one word are segmented; in those cases where the segmentation could be problematic (and this applies to several particular fragments of Old Welsh versification), the components of the phrases are explicitly cross-referenced; when the segmentation is unclear, or the reading is variable, the components of the phrase are given as a complete entry. Homographic/homophonic lexemes are treated under the different headings. Similar or identical instances which were analysed differently are normally considered separately. Parts of compounds as well as morphemes from nouns are not treated separately; their discussion can be found in the entries which contain the first element of the composite word.