This chapter reviews the literature on comprehension of media-based presentations to develop mental models of physical systems. It examines the representations and cognitive processes involved in understanding media-based presentations, the abilities and skills on which this understanding depends, and the effectiveness of different media for communicating different types of content. In reviewing how people construct mental models from media, it considers how people learn about the structure and functioning of physical systems from visual-spatial representations alone, including static and animated diagrams, and later reviews how they learn from combinations of visual-spatial and verbal representations. Iconic static diagrams can be effective for communicating the static structure of a system and can also be the basis for mental animation. Traditional print media, that is, static diagrams accompanied by text, can provide highly effective external representations to aid the development of mental models of dynamic systems.