This special issue revisits a classic topic in linguistic theory, A-bar movement, applied to developing and bilingual grammars. We claim that A-bar movement, or filler-gap dependencies, is still the quintessential linguistic phenomenon to illustrate the interaction between the biological endowment, the experience with language (past and present), and other cognitive considerations non-specific to the faculty of language (i.e. the three factors in language design discussed in contemporary Chomskyan approaches). These three factors are present in non-native and bilingual populations, in which asymmetries between grammatical knowledge and other factors are even more apparent, in such a way that we can observe the role of each of these components independently. The appearance of new data from unique populations of bilinguals and novel experimental methodologies justify the collection of articles gathered in this volume. These studies inform new and old theoretical debates about the accessibility to Universal Grammar (UG) in nonnative grammars, the relationship between the grammar and the parser, and the role of individual differences.