This combined text and manual presents the basic educational viewpoint of the Gesell Institute, that children should be entered in school (and consequently grouped and promoted) on the basis of their developmental or behavioral age, not on the basis of their chronological age or IQ. The introduction describes the research in which the Institute studied its theory. The main part of the book depicts the various aspects of the Institute's developmental test, describing its administration, evaluating the results, and defining its use in determining school readiness. Also, the results are used to demonstrate the concept of the existence of a specific age, such as 7-year-oldness. A section is devoted to parents, teachers, and administrators to help them understand the function of the testing and how it can help the child. A bibliography and several appendixes are included. (JS)