With the help of an innate language acquisition program, children typically enter the system of their mother tongue with astonishing confidence. This applies to both hearing and deaf children who acquire a sign language. The authors explain, based on linguistic theories and models, the principles governing the normal development of spoken and signed languages. They highlight both the parallels in the language acquisition processes and the differences in the language (acquisition) systems. In a previously unique manner, they connect insights from phonology and sign linguistics, thereby enhancing existing theoretical, diagnostic, and therapeutic approaches with concepts related to phonological fine structure. They illustrate their points with practical examples of children whose spoken language acquisition is hindered by hearing impairments or other reasons that cause it to deviate from the expected course.