This book investigates how semivowels were realized 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 and chronologically sensitive nature of this study, together with its 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.