This volume offers a discussion of the phonological and morphological development of Old Irish and its Indo-European origins. The emphasis is on the relative chronology of sound changes and on the development of the verbal system. Special attention is devoted to the origin of absolute and relative verb forms, to the rise of the mutations, to the role of thematic and athematic inflexion types in the formation of present classes, preterits, subjunctives and futures, and to the development of deponents and passive forms. Other topics include infixed and suffixed pronouns, palatalization of consonants and labialization of vowels, and the role of Continental Celtic in the reconstruction of Proto-Celtic. The final chapter provides a detailed analysis of the Latin and other Italic data which are essential to a reconstruction of Proto-Italo-Celtic. The appendix contains a full reconstruction of the Old Irish verbal paradigms, which renders the subject more easily accessible to a wider audience. The book is of interest to Celticists, Latinists, Indo-Europeanists and other historical linguists.