This article examines the knowledge of topic and subject particles in heritage speakers and L2 learners of Japanese and Korean. We assume that topic marking is mediated at the syntax-information structure interface, while subject marking pertains to narrow syntax. In comparing phenomena mediated at different levels of linguistic organization, we provide evidence for the hypothesis that information structure-level phenomena present greater challenges for bilingual speakers than those mediated within syntax. While these results may be interpreted as evidence of generalized interface-related deficits, we show that such a global explanation is not supported. Instead, a more nuanced account is developed, based on the recognition of different types of topic (anaphoric, generic, and contrastive) and different types of subject (descriptive and exhaustive). Under the proposed account, the non-native speakers’ deficits follow from three unrelated effects: the status of topic as an interface category, structural complexity, and the memory demands necessary for its interpretation in context.