This edited book is a collection of studies on protolanguage phonology, referring to the development of children’s autonomous linguistic systems from their first meaningful forms to complete cognitive and articulatory acquisition of language. The volume comprises chapters on child bilingual phonological development, understood as the acquisition or use of more than one linguistic code, whether actual languages, dialects, or communication modes, in an array of contexts. Such contexts include endogenous and exogenous bilingualism, heritage language, bilectalism, trilingualism, and typical and atypical use. The contributed works here will be of interest to researchers and postgraduate students investigating language acquisition in bi-/multilingual settings, as well as those working on child phonological development across a variety of languages.