This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book addresses a gap in the literature for clinicians who wish to develop their understanding of issues in the application of phonology to disordered speech. It provides an overview of current theories of phonological development and relates these to our understanding of phonological disorders in children. The book discusses what is required of a theory and evaluates six approaches: the structuralist, behaviourist, natural phonology, prosodic, cognitive and biological theories. It reviews the requirements of clinically applicable assessment procedures. The book also discusses the interactions between phonology and other aspects of language in phonological development and disorders. It reviews the literature available on the metalinguistic awareness of phonologically disordered children. The book deals with communicative or functional (in)adequacy and implications for the assessment and management of phonological disorders in children.