This paper discusses the interplay between acquisition and theory construction. It endeavors to show how a more direct and crucially bi-directional relationship between formal linguistic theory and the study of heritage language bilingualism can provide mutual benefit. It will be argued that data from acquisition—not exclusively but indeed especially from heritage language bilingualism—provide windows into the tenability of formal linguistic proposals. As a result, acquisition evidence—both developmental and in ultimate attainment—should be welcomed by all formal theorists, as they can confirm or disconfirm otherwise seemingly unverifiable theoretical arguments. Our claims will be illustrated by three research areas: language change, differences between functional and lexical features, and the interplay between core syntax and other components of the grammar.