There is growing evidence that the grammatical errors reflected in the speech of young children are often related to the nature of the input in the ambient language. Although theoretical frameworks differ in the degree to which input plays a role, there is acknowledgment that children require more input than previously assumed to resolve apparent grammatical ambiguities in the sentences they hear. In languages such as English, tense and agreement morphology seems especially at the mercy of correct interpretation of the input. This review discusses possible input-related reasons for the relatively late mastery of tense and agreement (and related) morphology in English-speaking children, and the especially protracted period of tense and agreement inconsistency in the speech of children with specific language impairment. It is noted that there is a reciprocal relation here. On the one hand, the children's failure to grasp particular syntactic structures can lead to the extraction and inappropriate use of verbs lacking tense and agreement. On the other hand, the tense and agreement (and related) morphemes in these structures can serve as important cues for interpreting the syntactic structure.