This study uses evidence from early English verse to reconstruct the course of some central phonological changes in the history of the language. It builds on the premise that alliteration reflects faithfully the acoustic identity and similarity of stressed syllable onsets. Individual chapters cover the history of the velars, the structure and history of vowel-initial syllable onsets, the behaviour of onset clusters, and the chronology and motivation of cluster reduction (gn-, kn-, hr-, hl-, hn-, hw-, wr-, wl-). Examination of the patterns of group alliteration in Old and Middle English reveals a hierarchy of cluster-internal cohesiveness which leads to new conclusions regarding the causes for the special treatment of sp-, st-, sk- in alliteration. The analysis draws on phonetically based Optimality-Theoretic models. The book presents valuable information about the medieval poetic canon and elucidates the relationship between orality and literacy in the evolution of English verse.