How do great athletes defy the power law of practice, according to which improvements in skill eventually plateau? Commentators and spectators alike are fascinated by how athletes such as Roger Federer, Tom Brady, and Serena Williams are capable both of moments of exquisite brilliance and of sustaining such excellence over a prolonged period. But what separates these great athletes from those who have achieved a certain level of expertise before finding that their performance has started to plateau? How is it possible for world-class athletes- athletes who seem to be at the top of their game- to keep improving? To solve this puzzle, this book presents a theory of