Drawing examples from a range of world languages, this textbook introduces the ways in which speech sounds become different over time. It explores how we produce and hear particular sounds and how overall word shapes and the pronunciation of individual words change. The roles of phonetics and phonology, morphology and syntax, traditional formal models and recent exemplar-based work in sound change are all examined. In covering both structural and societal issues, the book integrates different kinds of historical evidence and different theories into a coherent understanding of the full process of sound change.