Abstract:In recent years,explanations of developmental delays or inability to reach native-like attainment in specific areas of grammatical knowledge have been linked to the architecture of the language faculty.A recurrent claim is that linguistic properties at interfaces are inherently more "complex" than linguistic properties internal to a specific domain (syntax,phonology,semantics) due to the integration of different levels of linguistic knowledge/analysis.Even within interfaces,not all are created equal;some have been claimed to be even more complex,vulnerable,or more problematic than others.The author questions these recent claims about "vulnerability of specific interfaces" based on empirical evidence from recent studies on incomplete acquisition in L2 and heritage language acquisition,discusses examples of grammatical properties involving multiple interfaces and shows how the concept of internal and external interfaces is problematic.