Proto-Japanese is the reconstructed language stage from which all later varieties of Japanese, including Ryukyuan, descend. It has been studied both as an end in itself (as the genetic code of the Japanese language) and as part of endeavors to clarify the genetic affiliation of Japanese. Based on the state of the field, especially as represented in Samuel E. Martin's seminal work The Japanese Language Through Time (1987), this volume singles out key areas in the reconstruction of proto-Japanese where salient progress has been or promises to be made since Martin.

Contributions were invited from scholars working on the following areas: segmental phonology, use of dialect evidence, accent, morphology, and syntax. While the book first of all presents new research which advances our understanding of proto-Japanese, it also gives an overview over the state of the art in the field and its main issues.