What is wisdom? What does a wise person know? Can a wise person know how to act and live well without knowing the whys and wherefores of his own action? How is wisdom acquired? This Cambridge Element addresses questions regarding the nature and acquisition of wisdom by developing and defending the Expertise Theory of Wisdom in a systematic and dialectical way. Specifically, this theory argues that if a person S is wise, then (i) S knows that overall attitude success contributes to or constitutes well-being; (ii) S knows what the best means to achieve well-being are; (iii) S is reliably successful at acting and living well (in light of what S knows); and (iv) S knows why she is successful at acting and living well. The Element demonstrates how far a skill model of wisdom can go with respect to explaining various aspects of wisdom. Section 1 explains the nature of wisdom: Wisdom is a species of skill. Section 2 explains the structure of wisdom: Wisdom is bi-leveled. Section 3 explains the content of wisdom: A wise agent knows that overall attitude success matters most. Section 4 explains the norms involved in wisdom: A wise agent pursues the final goal whose specification is the most realistically, reproducibly, and/or challengingly achievable for the agent in question. Section 5 explains the acquisition of wisdom: Wisdom is acquired via feedback which is tied in with the rationally justified/justifiable references of correctness.