Predictions made by the self-control strength model were tested in a sample of underage social drinkers using ecological momentary assessment methodology. On days that participants experienced more self-control demands than average, they were more likely to violate their self-imposed drinking limit after controlling for mood and urge to drink. There was no relationship between self-control demands and urge or intention to drink, nor were self-control demands related to plans to limit drinking. When individuals planned to limit their alcohol intake, they were more affected by self-control demands than when they did not plan to limit their alcohol intake. Trait self-control moderated these relationships. Consistent with the self-control strength model, it appears that exerting self-control in nondrinking areas undermines individuals’ capacity to exert self-control of drinking in daily life.