Interface Delay is a theory of syntactic development, which attempts to explain an array of constructions that are slow to develop, which are characterized by being sensitive to discourse-pragmatic considerations of the type associated with the natural semantic class of definites. The theory claims that neither syntax itself, nor the discourse-pragmatic abilities related to executive function and theory of mind themselves are slow to develop. Rather, the claim is that the nexus or interface between the two cognitive domains is slow to develop. We review the development of subjects in child Spanish as an example of this delayed growth trajectory. Further, we review evidence that a delay in the development of tense causes concomitant delays in the seemingly unrelated phenomena of non-nominative case subject pronoun use and un-inverted wh- questions.