The key to the phonetic development of a given word across time lies in its accentuation. As the Old Celtic remains do not indicate their accent, it has to be deduced from their sound-changes. This book investigates for the first time in depth 14 Celtic phenomena resulting from the weakening of unstressed syllables (vocalic assimilations and reductions up to syncope, degeminations, ‚w‘-loss, nasal effacement, metathesis of liquids) and the strengthening of stressed ones (vocalic epenthesis and diphthongizations, geminations of consonants). The stress-patterns emerging from them are corroborated by the originally Celtic toponymy in today’s non-Celtic-speaking countries and allow us to reconstruct the Celtic subfamily of languages by drawing a much simpler accentual model which also finds typological support. With its diachronic discussion of more than 3,600 words the book represents, moreover, a big help in the understanding of Celtic lexicon and onomastics.