This book describes bilingualism from the perspective of the effect of a second language on the development of a first language, characterizing multilingualism as a dynamic and cumulative process that leads to a common underlying conceptual base (CUCB). The authors moreover frame their central hypotheses in the light of the most recent development of cognitive-pragmatic theories. These hypotheses run that multilingualism results in a linguistic capacity that differs from that of monolinguals; that symbiosis of a subconscious L1 top-down approach with the conscious L2 bottom-up approach positively affects language; and that practical, conceptual and cultural knowledge of an L2 improves the real-world knowledge of students.