Learning and speaking other languages provides new opportunities in life. However, achieving skill in a new language and multilingual ease is a slow and meandering process, and harder for some than for others. Why should this be? Surprisingly, research does not yet provide clear answers to that question. Language acquisition and multilingualism are complex phenomena, and yet are often studied as if single factors (e.g. age, socioeconomic status) could explain all variation found. Moreover, these phenomena are soften studied in separate disciplines with little mutual contact, leading to a fragmentation of knowledge. This partly explains the lack of clear answers. The research programme TEAM will therefore apply a transdiciplinary approach to study the complex interaction between linguistic, cognitive, neurological and social prerequisites that underpin language acquisition and multilingualism at the level of the individual and of social interaction. Empirically, we will examine language production and comprehension, motivation and linguistic social activities in children and adults, in typical and atypical populations, speaking one or several languages. Both behavioural and neurocognitive methods are applied. The ultimate goal is to propose an empirically based integrated, transdisciplinary model of variation in acquisition and multilingualism to generate new hypotheses and guide applied and translational research forward.