What is the best predictor of health, wealth, and success? Self-control. Humans have an unmatched ability to control their impulses. Yet we often fail to do so. Why do we fail to control our impulses—and what can we do about it? Much of my research has emphasized how our self-control capacity is powerful and fragile. When we control our impulses, it wears us out. And when we're fatigued, we struggle to meet our goals. I have applied this framework to understand why people behave aggressively, overeat, procrastinate, act selfishly, and make irrational decisions. I also use functional magnetic resonance imaging (fMRI) to identify how different types of impulsivity put people at risk for substance use problems.