This paper focuses on how phrasal prosody and function words may interact during early language acquisition. Experimental results show that infants have access to intermediate prosodic phrases (phonological phrases) during the first year of life, and use these to constrain lexical segmentation. These same intermediate prosodic phrases are used by adults to constrain on-line syntactic analysis. In addition, by two years of age infants can exploit function words to infer the syntactic category of unknown content words (nouns vs. verbs) and guess their plausible meaning (object vs. action). We speculate on how infants may build a partial syntactic structure by relying on both phonological phrase boundaries and function words, and present adult results that test the plausibility of this hypothesis. These results are tied together within a model of the architecture of the first stages of language processing, and their acquisition. (Contains 5 figures and 3 footnotes.)