Essential overload occurs when the amount of essential cognitive processing required by the multimedia instructional message exceeds the learner's cognitive capacity. This chapter examines the research evidence concerning three principles of multimedia design aimed at minimizing the effects of essential overload: the segmenting principle, the pre-training principle, and the modality principle. According to the cognitive theory of multimedia learning, the three ways to handle an essential overload situation are to allow the learner to slow down the pace of presentation (segmenting principle), provide the learner with knowledge that reduces the need for cognitive processing of the presentation (pre-training principle), or off-load some of the visual information onto the auditory channel (modality principle). The research reviewed in the chapter shows that instructional designers should be sensitive to working memory constraints when presenting a complex multimedia lesson.