Multi-word units are of particular relevance in language use: they are ubiquitous, highly frequent, and are characterized by the fact that they are not formed and analyzed anew with each use, but are produced and received as a whole and thus contribute to fluent and unobtrusive language processing. The present article examines the current state of research on the role of complex linguistic units in L2 acquisition. Of particular interest is the discussion of approaches according to which language acquisition occurs through language use.