The purpose of the eight studies reported in this paper was two-fold: to identify some of the relationships that exist between the unique properties of diagrams and various aspects of cognitive processes and learning, and, subsequently, to derive principles from these relationships that would direct the design and use of diagrams in the classroom. The unique properties of diagrams can be discussed in four different contexts, and these studies examined hypotheses developed within all four contexts. The conclusions from these studies can be summarized as follows: (1) diagrams help learners because they direct attention to important information, replacing critical verbal information with graphic devices such as lines and arrows; (2) diagrams help low-verbal learners overcome some of their difficulty with language by providing information in a form they can handle more easily; (3) through the use of normal left-right, top-bottom layout, arrows and other graphic devices, diagrams can teach sequences of events effectively; (4) the addition of study questions to diagrams helps learners by directing their attention to critical information; and (5) prompting can be useful in helping learners. Eight practical design principles were derived from the studies for use in designing and using diagrams. Fifty-seven references are listed. (Author/LLS)