We expand upon a previous proposal by Bloom and Wynn (1997) that young children learn about the meaning of number words by tracking their occurrence in particular syntactic environments, in combination with the discourse context in which they are used. An analysis of the Childes database (MacWhinney, 2000) reveals that the environments studied by Bloom and Wynn (specifically, the partitive frame x of the y) do not on their own distinguish between number terms and those terms that are more generally quantity denoting. A set of novel word-learning experiments reveals that children (and adults) are aware of the semantic constraints of two of the syntactic environments targeted by Bloom and Wynn (the partitive frame and modification by very) but either rely upon or benefit from contextual information supporting learning where a number word can but need not be used in a sentence. We propose that children most likely combine their knowledge of counting principles (Gelman & Gallistel, 1978) with the discourse context to support the conclusion that a number word can appear in certain syntactic frames. Overall, the results indicate that recruiting syntax-semantics knowledge and assigning a number-word meaning to a new word is a delicate affair, even for adults, and suggest that there is a tight link between surface-level form, underlying constraints, and the discourse context in number word learning. (Contains 8 tables, 3 figures, and 7 footnotes.)