This chapter examines how people use diagrams, models, animations, and multimedia presentations to learn about physical systems. Comprehension of these representations is analyzed within an information processing framework that specifies the cognitive processes involved in understanding external representations, including attention, encoding, inference, and the integration of different representations. The chapter first reviews the literature on the comprehension of physical systems from diagrams, animations, and models (both physical and virtual) alone and then reviews how people understand physical systems from multimedia presentations that combine these media and augment them with verbal instruction. The picture that emerges is that comprehension of media is an active process of knowledge construction rather than a passive process of internalizing the information presented in an external display. Because multimedia understanding depends on active information processing, it can be influenced considerably by the abilities, skills, and knowledge of the student. What is learned from media is jointly determined by aspects of the display and aspects of the learner.