Recent research on child and adult multilingualism has focused on a range of interface conditions on syntactic realization. I will discuss two examples. First, pronouns and other referring expressions require efficient integration and updating of context-dependent information: these structures develop late in monolingual children, remain variable in proficient second language speakers, and become unstable in speakers experiencing native language attrition. Second, the selection of perfective auxiliaries avere/haben (‘have') and essere/sein (‘be') in Italian and German systematically varies along a gradient hierarchy defined by the aspectual type of the intransitive verb and the context in which the verb appears: the use of auxiliaries with variable verbs is sensitive to the cognitive capacity of individual speakers to integrate linguistic and non-linguistic information outside the verb itself, and presents similar developmental patterns to pronouns in late bilingualism. I will argue that an account of these facts needs an interdisciplinary approach that considers the interaction of linguistic and general cognitive factors.