Frisian is a West Germanic language, nowadays spoken in a number of varieties, distributed across the Netherlands and Germany. Arjen P. Versloot and Michiel de Vaan describe the reconstructed common ancestor of all attested varieties of Frisian, namely Proto-Frisian. It developed from a shared Northwest Germanic ancestor in the aftermath of the Migration Period that marked the transition to the early Middle Ages, and was closely related to Old English, Old Saxon and Old Norse. The focus of the book is on the reconstruction of the phonological and morphological changes that occurred between Northwest Germanic and Proto-Frisian. The incorporation of synchronic runic evidence from the North Sea region enables the establishment of an absolute chronology of the changes. The linguistic developments are interpreted by drawing on recent insights from archaeological and archaeogenetic research.