This book is an investigation of how semivowels were realised in Indo-European and in early Greek. More specifically, it examines the extent to which Indo-European *i and *y were independent phonemes, in what respects their alternation was predictable, and how this situation changed as Indo-European developed into Greek. The comprehensive nature of this study, its chronological sensitivity, and careful assessment of what is inherited and what is innovative, enables substantive conclusions to be drawn regarding the behaviour of semivowels at various stages in the history of Greek and in Indo-European itself.