This book studies Dutch-English-Mandarin trilinguals to answer research questions about trilinguals' mental lexicon representation pattern, routes for L3 conceptual access, and the role of instruction language herein. To gain scientific and reliable results, interviews, questionnaires and experiment series were all adopted for different purposes. Besides, this study compares trilinguals with lower and higher L3 levels to map the developmental trajectories of trilinguals' mental lexicon representation and L3 conceptual access. The current research proposes a three-stage hypothesis and a parabola-shaped model, to explain the role of L2 as instruction language throughout L3 learning process and its contrast with the role of L1. Moreover, Trilinguals' All-Connected Models of mental lexicon representation and L3 conceptual access are the first attempts to give detailed descriptions of trilinguals' mental lexicon representation and L3 conceptual access from both static and developmental perspectives. Furthermore, it sheds light on foreign language teaching and learning practice, and contributes to further improving overseas Chinese language teaching and culture spreading performances, thus fulfilling its theoretical, methodological and practical values.