During the past decades Balto-Slavic accentology has become increasingly important for the reconstruction of Proto-Indo-European. This study tries to provide an explanation for the phenomenon of metatony in Baltic, i.e. the phenomenon that in certain Baltic forms a morpheme shows the reflex of the Balto-Slavic circumflex intonation where we would expect the reflex of the acute intonation (métatonie douce) or vice versa (métatonie rude). The subject necessarily involves an inquiry into the origin of the Lithuanian and Latvian tone systems. Furthermore, it requires the assessment of a large number of etymologies. In the final chapter of the book, the developments which are considered to be relevant to the rise of metatony are incorporated into a relative chronology. The investigation is based on a comprehensive collection of data, including evidence from Lithuanian and Latvian dialects and Old Lithuanian. In comparison with earlier studies on the subject, the Latvian evidence plays an essential role. This book tries to demonstrate that the value of Latvian data for Balto-Slavic accentology has hitherto not been fully recognized.