Strong correspondences have been found between symbol development in play and language in three aspects: (1) correlations in frequency and rate, (2) overlap in referential content, and (3) parallels in qualitative levels and sequences of development. Within language, a developmental progression has been found from what is termed “nonreferential” to “referential” uses of words. The nonreferential words are not names for actions or entities; rather, they are procedures that are used in restricted contexts that may include particular actions or entities. At each developmental level, the difference between substantives and function words has to do with the kinds of referents—entities, events, relationships—involved in that language game or procedure. However, both kinds of words are in themselves functions. The field of child language research has been divided regarding: (1) the developmental levels of word use—that is, in terms of contextual freedom, (2) the kinds of features that predominate in the rules for using words, (3) the structure of the categories that underlie word use, and (4) individual differences in the things that children want to accomplish with words.