Using information drawn from research studies, the literature, and personal experiences, selected physiological, cognitive, and affective differences among 10- to 15-year-old middle school science students are illustrated. A rationale is presented for K-12 school organization based on human brain growth studies, with particular emphasis on middle school organization; implications for science curriculum and instructional practices based on these studies are suggested. It is indicated that the characteristics of middle school students be given high priority during examinations of middle school curricula and instructional practices. (Author)