Abstract The planning fallacy refers to a prediction phenomenon, all too familiar to many, wherein people underestimate the time it will take to complete a future task, despite knowledge that previous tasks have generally taken longer than planned. In this chapter, we review theory and research on the planning fallacy, with an emphasis on a programmatic series of investigations that we have conducted on this topic. We first outline a definition of the planning fallacy, explicate controversies and complexities surrounding its definition, and summarize empirical research documenting the scope and generality of the phenomenon. We then explore the origins of the planning fallacy, beginning with the classic inside–outside cognitive model developed by Kahneman and Tversky [Kahneman, D., & Tversky, A. (1979). Intuitive prediction: biases and corrective procedures. TIMS Studies in Management Science, 12, 313–327]. Finally, we develop an extended inside–outside model that integrates empirical research examining cognitive, motivational, social, and behavioral processes underlying the planning fallacy.