This paper reviews research and opinions concerned with the lateralization of brain functions and cognitive development as described by Piaget. Optimal cognitive functioning in humans is a product of the complete development of interhemispheric communication in the brain. Growth spurts in brain development have been found to correlate closely with Piaget's developmental stages. The preoperational stage includes the progressive development of the neural fibers which connect the brain hemispheres, i.e., the corpus callosum (CC), and includes a brain growth spurt between the ages of 2 and 4. At this stage true lateralization of cognitive functions begins to occur as both hemispheres and the interhemispheric communication system continue to develop. Both right and left hemispheric developments can be seen in the stage of concrete operations. The development of the CC appears to be dependent upon its use and on the utilization of both hemispheres. A bias toward linear, logical thinking, or left hemisphere rationality, predominates in our educational system. This emphasis appears deficient insofar as hemisphere interaction has been associated with optimal cognitive functioning, increased creativity, self-actualization and mental health, as well as specific learning outcomes. Piagetian classroom practices for optimal development utilize both brain hemispheres. (RH)