This monograph reports a longitudinal study of the language development of 211 pupils as they progressed from kindergarten through twelfth grade. Data include transcribed interviews, written compositions, reading and IQ scores, listening scores and ratings, teachers' ratings on specified language factors, and reports of books read. Results concentrate on differences among three subsamples: 35 pupils rated high in overall language ability by their teachers, 35 rated low, and 35 selected randomly from the total group. Differences between pupils who use language effectively and those who do not are described, as are the stages, velocity, and relative growth of the children's language during the 13 years of the study. Appendixes provide samples of transcripts, compositions, instruments, and methods of analysis. (AA)