The view that a bilingual speaker, or a speaker acquiring more than one language, is the sum of two —or more— monolinguals is proving to be a myth rather than a reality. Accordingly, this book provides a new profile of children and young people becoming bilingual or multilingual in today’s multicultural Spain. The chapters present studies on the acquisition of the four official languages plus the languages of several new communities. They include descriptive, functional, pragmatic and formal perspectives, covering phonetics, lexis, morphology and syntax, as well as code mixing and input, bilingual twins, SLI bilingualism, narratives, literacy, age and stay abroad effects. The book should be of interest to graduate students and researchers working in the field of second and foreign language acquisition and multilingualism, language planners, language teachers and families alike.