How can we help students understand scientific systems? One promising approach involves multimedia presentations of explanations in visual and verbal formats, such as presenting a computer-generated animation synchronized with narration or on-screen text. In this paper, we present a cognitive theory of multimedia learning from which the following six principles of instructional design are derived and tested: the split-attention principle, the spatial contiguity principle, the temporal contiguity principle, the modality principle, the redundancy principle, and the coherence principle.