Recent developments in neuroscience and social science are illuminating the critical importance of regret in human choices, including criminal decision making. After differentiating regret from related emotions (e.g., disappointment, sadness, shame), I argue that regret can prompt desistance from crime and that regret avoidance is a powerful mechanism of conformity. I then turn to American and European penal history to demonstrate that the invention of the prison was premised on the notion that solitary confinement could inculcate regret in prisoners and thereby change them profoundly. The place of regret in both historical and emerging thinking about human choice underscores its importance for understanding the connections between emotions and criminality.