Phonemic awareness and phonetic skill are the backbones of phonological theory. In phonological acquisition, the presence or lack of the former crucially determines the outcome of the latter. This inescapably becomes a common thread that interweaves developmental phonology in both childhood and adulthood. Child and adult-learner speech in the course of development constitute separate linguistic systems in their own right: they are intermediate states whose endpoint is, or ought to be, mastery of targeted speech either in a first or a second language. These intermediate states form the theme of the proposed book that introduces the term protolanguage (to refer to child language in development) and juxtaposes it with interlanguage (to refer to language development in adulthood). Though major languages like English and Spanish are not excluded, there is an emphasis in the book on under-reported languages: monolingual Hungarian and Swedish and bilingual combinations, like Greek-English and German-English. There is also an emphasis on under-represented studies in IL: L2 German from L1 French; L2 English from Catalan and Portuguese; and in dialectal acquisition of Ecuadorian Spanish from Andalusian speakers. This volume brings together different methodological approaches with a stress on both phonetic and phonological analysis. The volume has a focus on links between developmental perspectives in protolanguage (introduced in a specific way here) and interlanguage (typically associated with second language development in the post-critical-period). Its strengths are that it includes: both child and adult developmental perspectives; studies on less-researched languages and combinations of languages; descriptive and/or theoretical results from a combination of methodological approaches (e.g. single-case, cross-sectional; spontaneous speech samples, narrative retells); a consideration of speech acquisition in the general context of language. The central theme underpinning the volume is hoped to motivate a shift in the general tendency among researchers to specialize in language subfields (L1 acquisition; L2 acquisition, bilingualism; typical/atypical language) of what is actually one common linguistic domain, i.e. the study of speech sounds (phonology/phonetics).