This book studies the acquisition, loss and re-acquisition of Spanish, English, Portuguese, and Hebrew, the first languages of this writer's son. It applies the results of current work in the areas of psycholinguistics, bilingualism, and applied linguistics to the study of language development in one multilingual child, Noam, from birth to age 17. The acquisition, loss, and re-acquisition of four languages by Noam also is compared with that of other children studied by the author and others. This book uncovers linguistic, cognitive, psychological, and social mechanisms of language acquisition, loss, and re-acquisition and documents the child's increasing, decreasing, and, in turn, increasing proficiency in four languages. This book applies Dromi's guidelines for qualitative case-study research to the study of language development in one multilingual child (Spanish, Portuguese, Hebrew, English), Noam, from birth to age seventeen. In addition, the results of Noam's case study are compared with other case studies conducted by this author as well as by other researchers.