Humans use their linguistic knowledge in at least two ways: on the one hand, to convey what they mean to others or to themselves, and on the other hand, to understand what others say or what they themselves say. In either case, they must assemble the syntactic structures of sentences in a systematic fashion, in accordance with the grammar of their language. In this article, we advance the view that a single mechanism for building sentence structure may be sufficient for structure building in comprehension and production. We argue that differing behaviors reduce to differences in the available information in either task. This view has broad implications for the architecture of the human language system and provides a useful framework for integrating largely independent research programs on comprehension and production by both constraining the models and uncovering new questions that can drive further research.