This outstanding volume presents a state-of-the-art overview of linguistic research into the acquisition of phonology. Bringing together well-known researchers in the field, it focuses on constraints in phonological acquisition (as opposed to rules), and offers concrete examples of the formalization of phonological development in terms of constraint ranking. The first two chapters situate the research in its broader context, with an introduction by the editors providing a brief general tutorial on Optimality Theory. Chapter two serves to highlight the history of constraints in studies of phonological development, which predates their current ascent to prominence in phonological theory. The remaining chapters address a number of partially overlapping themes: the study of child production data in terms of constraints, learnability issues, perceptual development and its relation to the development of production, and second-language acquisition.