This article is concerned with the correspondence conditions that hold between certain semantic relations—including part-whole relations, possession, location, and the semantic features [± animate] or [± count]—and certain syntactic structures including genitives and relative clauses. The objective is to determine the extent to which these correspondence conditions derive from Universal Grammar or are “learned” by children in response to input from caretakers and others. Interface conditions imposed by Universal Grammar (UG) are expected to appear early in the course of language development despite the vagaries of the primary linguistic data. In this article, we show that children as young as 3 years old adhere to specific semantic distinctions and to specific constraints on the mapping of these distinctions onto syntactic structures. Moreover, the children show more stringent adherence to interface conditions than adult speakers of the same languages do, indicating that children's mapping relations between syntax and semantics are not based on their experience but rather are projected from UG.