Abstract This book traces the development of Proto-Indo-European into Ancient Greek of about the 5th century bc, attempting to recover the relative chronology of changes whenever possible and to explore in detail how the Ancient Greek dialects diversified. It is roughly parallel to From Proto-Indo-European to Proto-Germanic (2nd ed.; Oxford University Press, 2017) and is intended for readers who have at least begun to learn Ancient Greek and are interested in its prehistory. Readers should be aware that this book is not a complete historical and comparative grammar of Greek.