THE MATERIAL sampled in this chapter has been limited, first, to research studies, and second, to such of these studies as seem to have implications for educational procedures. The general scope of the chapter parallels the three most commonly designated educational levels of elementary, secondary, and higher education but includes three additional classifications warranted by recent research, namely preschool, general adult, and armed services. Preschool 1 The authors are greatly indebted to the following members of the graduate seminar in educational psychology who surveyed the pertinent literature as a joint enterprise :