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. Instead, materials should be formatted so that disparate sources of information are physically and temporally integrated, thus obviating the need for learners to engage in mental integration. Eliminating the need to mentally integrate multiple sources of information reduces extraneous cognitive load and frees resources for learning. This chapter provides the theoretical rationale, based on cognitive load theory, for the split-attention principle, describes the major experiments that establish the validity of the principle, identifies the conditions under which it is most likely to occur, and indicates the implications for instructional design involving multimedia materials.