Thesis: Ph. D., Massachusetts Institute of Technology, Department of Brain and Cognitive Sciences, 2018.Cataloged from PDF version of thesis.Includes bibliographical references (pages 188-214).Research in the domain of cognitive science has tended to neglect emotions. In my thesis, I take several steps to fill this gap by looking at people's representation of emotions, and its connection to other representations typically studied in cognitive science. I argue that people have an intuitive theory of emotion that is causally intertwined with their understanding of the physical and social world broadly. This intuitive theory allows us to use observed emotional cues as a window, to recover unobserved information about the world. I study these abilities in both adults and children, to gain insight into the most fundamental representations supporting such abilities. I also use computational models to capture the hierarchical, causal structure of this intuitive theory of emotion. In Study 1, I show that infants as young as 12-17 months can discriminate diverse within-valence emotional expressions elicited by funny, exciting, adorable, delicious, and sympathetic events, and map them onto their probable causes. In Study 2.1, I present that preschoolers can recover rich mental state information from observed emotional expressions. When the valence of someone's face changes between anticipated and actual outcomes, children by five gain insight into what she wants and believes about the world. Study 2.2 bridges theory of mind research, accounts of emotion attribution, and formal modeling, to provide a formal account of how people jointly infer beliefs and desires from emotional expressions. Study 3 tests children's understanding of social display rules. By middle childhood, children can use one person's emotional expressions regulated by a social context to infer the mental states of another. Altogether, these findings suggest that emotional cues provide a valuable entrée into the unseen world. Not only adults, but also children, can use observed emotional expressions to infer their external causes and the internal mental states of other people. Although this intuitive theory of emotion may not necessarily mirror the actual processes of how emotions are generated, it supports rational inferences much of time, and it may be formed early in development. I see this work as bridging gaps across disciplines and helping advance the cognitive science of emotion understanding."Funding support, including Leventhal, Stark, and Henry E. Singleton Fellowships, and the Center for Brains, Minds and Machines (funded by NSF STC Award CCF-1231216)"--Page 5by Yang Wu.Ph. D.