The redundancy principle (or redundancy effect) suggests that redundant material interferes with rather than facilitates learning. Redundancy occurs when the same information is presented concurrently in multiple forms or is unnecessarily elaborated. According to cognitive load theory, coordinating redundant information with essential information increases working memory load, which may interfere with learning. Eliminating such redundant information removes the requirement to coordinate multiple sources of information. Accordingly, instructional designs that eliminate redundant material can be superior to those that include redundancy. This chapter summarizes research and theory concerned with the effect of processing redundant information in multimedia learning, a history of research in instructional redundancy, the conditions of applicability of this principle, and its instructional implications.