A growing body of research with typically developing children has begun to show that the acquisition of grammatical morphemes interacts not only with a developing knowledge of syntax, but also with developing abilities at the interface with prosodic phonology. In particular, a Prosodic Licensing approach to these issues provides a framework for understanding why children's early use of grammatical morphemes is so variable. This in turn provides a means for making predictions, given the prosodic structure of a particular language and the location of a particular grammatical morpheme, if it is likely to be produced or not. Given that many of the patterns of development found in typically developing children are found in older children with Specific Language Impairment (SLI) as well, the Prosodic Licensing Hypothesis should provide a better understanding of some of the variable use of grammatical morphemes found in children with SLI.