This volume offers an ontogenetic perspective on research on L3, multilingualism and multiple languages acquisition and a conceptually updated picture of multilingualism studies and third/multiple language acquisition studies. The contributions by prominent scholars of multilingualism present state-of-the-art accounts of the significant aspects in this field. This unique collection of articles adopts a broad-spectrum and synthesized view on the topic. The volume, largely theoretical and classificatory, features main theories, prominent researchers and important research trends. The articles also contain factual and historical material from previous and current decades of research and offer practical information on research resources.