The 13-year study reported in this monograph attempted to accumulate a mass of longitudinal data on every aspect of linguistic behavior in 211 students representing a cross-section of American urban children. Presented as the major objectives of the study, which followed the subjects from kindergarten through grade 12, are 11 hypotheses to be tested dealing with whether the growth in children's language follows a predictable sequence, whether definite stages of language development can be identified, and whether the velocity and relative growth in language ability can be ascertained and precisely predicted. Also reported are the results of research to develop fundamental methods of analysis to aid the scientific study of children's language. The data presented in extensive tables and charts were obtained from annual oral interviews and their transcripts, written compositions, reading tests, I.Q. tests, listening tests, tests in the use of subordinating connectives, teachers' ratings, lists of books read, and miscellaneous other sources. Results are reported on the investigations into basic linguistic measures (e.g., average number of words per "communication unit" and "maze"), weighted indexes of elaboration, transformational analysis, verbs, and dependent clauses. (LH)