The volume espouses an ecosystemic standpoint on multilingual acquisition and learning, viewing language development and use as both ontogenesis and phylogenesis. Multilingualism is inclusively used to refer to sociolinguistic diversity and pluralism. Whether speech, writing, gesture, or body movement, language is a conduit that carries meaning within a complex, fluid, and context-dependent framework that engages different aspects of the individual, the communicative interaction, communicative acts, and social parameters. Continually modified over the years to better represent its multidisciplinary scope, the sociobiological notion of language has found steady and productive ground within major theoretical frameworks, which, individually or holistically, contribute to a rounded understanding of language acquisition, learning, and use by exploring both system-internal and system-external factors and their interaction. Summoning the work of leading academics, the volume outlines the changing dynamics of multilingualism in children and adults internationally with the latest advances and under-represented coverage that highlight the ecosystemic nature of multilingual acquisition, learning, and use.