This book presents a comprehensive and critical overview of historical phonology as it stands today. Research from every part of the field is examined from a variety of theoretical perspectives and drawing on data from a wide range of languages. The book begins by considering key current research questions, the early history of the field, and the structuralist context for work on sound change. In the second part, chapters examine evidence and methods, including phonological reconstruction, typology, and computational and quantitative approaches. Part III looks at types of phonological change, including stress, tone, and morphophonological change. Part IV explores a series of controversial aspects within the field, including the effects of first language acquisition, the mechanisms of lexical diffusion, and the role of individuals in innovation. Part V considers theoretical perspectives on phonological change, including those of evolutionary phonology and generative historical phonology. The final part examines sociolinguistic and exogenous factors in phonological change, including the study of change in real time, the role of second language acquisition, and loanword adaptation.