A broader approach to research in hu- j man development is proposed that focuses on the pro- \ gressive accommodation, throughout the life span, between the growing human organism and the changing environments in which it actually lives and grows. \ The latter include not only the immediate settings containing the developing person but also the larger social contexts, both formal and informal, in which these settings are embedded. In terms of method, the approach emphasizes the use of rigorousj^d^igned exp_erjments, both naturalistic and contrived, beginning in the early stages of the research process. The chang- ing relation between person and environment is con- ceived in systems terms. These systems properties are set forth in a series of propositions, each illus- trated by concrete research examples.