It has long been known that children may use a particular grammatical morpheme inconsistently at early stages of acquisition. Although this has often been thought to be evidence of incomplete syntactic representations, there is now a large body of crosslinguistic evidence showing that much of this early within-speaker variability is due to still developing phonological and/or prosodic representations. This article reviews recent research showing how a phonological approach to the emergence of grammatical morphology makes it possible to make crosslinguistic predictions about what will be acquired early and what acquired late. This is known as the Prosodic Licensing Hypothesis, providing a unified framework for understanding the course of morphosyntactic development across languages. The implications are both theoretical and methodological, suggesting that children may know more about the grammar of their language at an earlier age than is often assumed, and that this can only be revealed by taking prosodic phonology into account in designing early tests of syntactic development.