Gesell, the leading authority on child development, was bom in Wisconsin and graduated from the State University. After receiving his Ph.D. degree from Clark University in 1906, he taught psychology at Los Angeles State Normal School. In 1911 he was appointed professor of education at Yale, founding the Yale Psycho-Clinic (later changed to Yale Clinic of Child Development) in the same year. In 1915 he obtained a medical degree at Yale Medical School, and taught child hygiene there until 1948. The Gesell Institute of Child Development was established in his honor in 1915, and he was active as research consultant until 1958. During his long professional career, these centers trained many pediatricians and child psychologists who practiced their profession all over the world Gesell’s voluminous bibliography contains over 400 items, including among others, papers on cerebral palsy, mongolism, cretinism, visual deviations, and studies of twins. Many of his earlier works dealt with investigations of infancy and the preschool years. His major books in this field are The Mental Growth of the Preschool Child (1925), Infancy and Human Growth (1928), Infant Behavior (1934), Developmental Diagnosis (1941) and Infant and Child in the Culture of Today (1943). Later he turned his attention to the development of the child during the school years, publishing The Child from Five to Ten in 1946 and Youth: the Years from Ten to Sixteen in the same year. His last book was Vision: Its Develoment in Infant and Child, published in 1949. See ADOLESCENCE (THEORIES).Gesell’s major contributions lie in the study of changes in behavior patterns from the fetal period through adolescence. In his opinion, behavior develops in a predictable and measurable sequence, although the exact timing is not identical from child to child. This view is based on a belief that the organism goes through a cycle of “morphogenetic events” that enable it to progress in an orderly way in terms of the activities it can perform. He held that emerging behavior patterns have as much structure as does the growing physical organism. In describing the process of development, he stressed an alternation of ages of flexion and extension, inwardness and outwardness, equilibrium and disequilibrium, which he believed to be in accord with a principle of reciprocal neuromotor activity. Thus the child has more internal tensions at age three than at two, and is more unstable at thirteen than at fourteen or sixteen. These sequential changes meant that “behavior has shape.” He believed this shape is basically determined by internal forces rather than by environmental factors, which in his opinion can only modify, inflect, and support the progressive changes that occur. This emphasis on maturation was not readily accepted, since it was formulated at a time when behaviorism had come to the fore and was taking an extreme environmentalist position on the development of the personality. Most of Gesell’s research on infant development was carried out in a controlled test situation. He placed the infants in a specially contructed dome, and filmed their spontaneous activities as well as their reactions to various objects and stimuli such as a bell or a pellet. He and his associates would make a detailed behavior analysis of the films and other records, comparing the results with findings on other normal subjects. Many of the films were made available to other universities and child study centers for teaching and demonstration purposes. Gesell was also one of the first investigators to make use of one-way screen observations and the “co-twin control” method. The latter technique was used to test the effects of learning versus maturation— for example, he would allow one identical twin to go through a learning experience, such as climbing steps, and a few weeks later compare his performance with that of the other twin to see if maturation took place during those weeks. He later extended his investigations to the less structured situations of the home and schoolroom. See TWINS. Gesell’s influence was, and is, extremely pervasive, but it is too early to evaluate its full effect. The Gesell Developmental Tests are given at many clinics, and are especially useful in assessing neurological defect. Another direct effect of his work is the inclusion of a section on the development of infant behavior in the admittance examination of the American Academy of Pediatrics. But the best indication of his influence is the fact that his books have been widely used as texts in colleges and teacher-training schools, as well as by the public itself, and his sequences of development are constantly cited by authorities in psychology and psychiatry, although many investigators put more emphasis on variations and individual differences than did Gesell. Beyond this, we can say that a great many specialists agree with the comment made by the Yale Register at the time of his death: “Dr. Gesell was a pioneer, one who traced unchartered paths to chartered conclusions