In this paper, we bring to the attention of the linguistic community recent research on heritage languages. Shifting linguistic attention from the model of a monolingual speaker to the model of a multilingual speaker is important for the advancement of our understanding of the language faculty. Native speaker competence is typically the result of normal first language acquisition in an environment where the native language is dominant in various contexts, and learners have extensive and continuous exposure to it and opportunities to use it. Heritage speakers present a different case: they are bilingual speakers of an ethnic or immigrant minority language, whose first language often does not reach native-like attainment in adulthood. We propose a set of connections between heritage language studies and theory construction, underscoring the potential that this population offers for linguistic research. We examine several important grammatical phenomena from the standpoint of their representation in heritage languages, including case, aspect, and other interface phenomena. We discuss how the questions raised by data from heritage speakers could fruitfully shed light on current debates about how language works and how it is acquired under different conditions. We end with a consideration of the potential competing factors that shape a heritage language system in adulthood.