This volume provides a multifaceted view of certain key themes in multilingualism research today and offers future directions for this research area in the context of the multilingual development of individuals and societies. The selection of studied languages is eclectic (e.g. Amondawa, Cantonese, Bulgarian, Dene, Dutch, Eipo, Frisian, German, Mandarin Chinese, Māori, Russian, Spanish, and Yukatek, among others), they are typologically diverse, and they are contrasted from a variety of perspectives, such as cognitive development, aging, acquisition, grammatical and lexical processing, and memory. This collection also illustrates novel insights into the linguistic relativity debate that multilingual studies can offer, such as new and revealing perspectives on some well-known topics (e.g. colour categorisation or language transfer). The critical and comprehensive discussions of theoretical and methodological considerations presented in this volume are fundamental for numerous current, future, empirical and interdisciplinary studies of linguistic diversity, linguistic typology, and multilingual processing.