One dominant scientific view holds that willpower is a type of muscle which can be weakened through use in the short term and strengthened through use over time. However, evidence from neuroscience, social psychology and behavioral economics suggest that willpower is regional, subverted through desire and strengthened by strategy--these are features a muscular account would not predict. It is better to think about willpower as a skill with a physiological component. Willpower strategies extend the brute effort of self-control through the use of reason and have the practical effect of increasing self-regulation. Willpower is "worth wanting" because there is a gap in our given desires and our evaluations. In general willpower is the skill responsible for extending the motivational force of evaluations to overcome the motivational force of other interests. Of course, willpower can be used in the service of evil, but in general it is a power we would prefer to have. Interestingly, not all cases of weakness of will are, on balance, bad. As a practical matter weakness of will is a crucial element of developing willpower skills over time. Just as a skilled batter relies on failures to teach what is required for good hitting, willpower failures are an important element in developing habits for success. Additionally, the motivational failure of evaluation built in to weakness of will requires a commitment to practical claim that one can choose how to act in ways not dictated by given desires. This commitment to the importance and viability of evaluation is a crucial component of having a moral perspective in a natural system and weakness of will is a signifier of this foundational element of a practical perspective.