What was unique about our laboratory was the detailed simulation of a human rearing environment for infant chimpanzees, together with continuous, intensive observation and recording of behavioral development. Communication in American Sign Language (ASL) was integrated into this procedure in a way that simulates the teaching of vocal and gestural languages to human children. The result is a comprehensive longitudinal record of the stage by stage development of two-way communication in sign language and its relation to basic aspects of behavioral development such as: locomotion, manipulative skills, observational learning, and social play. In the longitudinal records of five infant chimpanzees, we can trace the patterns of development and also examine the consistency of individual differences through the first five years. The central question is the relation between developing skills in the use of American Sign Language and the rest of behavioral development. To analyze factors that influence development, we have compared records for infant chimpanzees reared in the cross-fostering laboratory with records for chimpanzees reared in the wild by their own mothers and chimpanzees reared in cages under conventional laboratory conditions, as well as with records of development for human children, reared in homes and reared in institutions.