This chapter reviews research on cognitive development during adolescence, with particular emphasis on biological and experiential influences on gender differences in development. The strong consensus is that sex-related differences increase with age and that early adolescence, when pubertal change occurs, is the crucial time period for the substantive differentiation of the sexes in performance on spatial tasks, with males performing better on most tasks. The development of sex differences in cognition, often thought to merge in adolescence, has been attributed to a variety of influences. A common explanation for behavioral changes seen in adolescence is that puberty causes great upheaval in adolescent behavior. A clear factor in success on cognitive tasks is experience. It has been proposed that experience leading to skill development is influenced by genderrole stereo-types and expectations. The substantial evidence that neurological development continues at least until puberty indicates that plasticity in development may also occur until this time.