When people solve mechanical problems, such as troubleshooting, designing, or understanding the operation of mechanical systems, they typically operate on visual displays of these systems—either the machine itself or a schematic diagram of the machine. If the display is central to their problem-solving process, we can assume that their eye fixations on the display indicate the information that they are processing at each stage of the problem-solving process (Just & Carpenter, 1976). Given this assumption, eye-fixation data has provided important insight into cognitive tasks such as mental rotation (Just & Carpenter, 1976, 1985), sentence-picture verification (Just & Carpenter, 1976) and visual analogies (Carpenter, Just, & Shell, 1990). This chapter presents research in which eye-fixation data was used to study a more complex cognitive task, the task of understanding a mechanical system.