This article focuses on the description of one of the main features of current multilingualism, complexity, through a selection of issues related to its role in L2 acquisition, as the proper notion of multilingualism, multilingualism as a social phenomenon and multilingualism as a multidimensional phenomenon. We also present several aspects of multilingualism in its relationship to L2 acquisition, notably some involved in the notion of multilingualism as a holistic process, and the contexts of multilingual language acquisition, and illustrate new findings of multilingualism in these areas, both from a theoretical and research perspective.