The split-attention principle states that in the design of instruction, including multimedia instruction, it is important to avoid materials that require learners to split their attention between, and mentally integrate, multiple sources of information. The sources of information should be both physically and temporally integrated in order to reduce unnecessary search for referents and so reduce extraneous cognitive load. Whether sources of information are intelligible in isolation, and whether the information is high in element interactivity, depends not only on the instructional material, but also on learner characteristics. Cognitive load theory, which gave rise to the split-attention principle, which is based on an understanding of human cognitive architecture, especially the relations between working and long term memory, provides theory-based and experimentally tested instructional guidelines. Those guidelines that are associated with the split-attention effect and that have been discussed in this chapter have the potential to substantially improve multimedia instruction.