Throughout his corpus, Kant repeatedly and resolutely denies that there is a duty to promote one's own happiness, and most present-day Kantians seem to agree with him. In this book, I argue this denial rests on two main ideas: (1) a conception of duty that makes the principle of ought implies can (OIC) and the principle of alternate possibilities (PAP) analytic, and (2) the claim that humans necessarily promote their own happiness. The book defends OIC and PAP but nonetheless attacks (2), and it supplements this attack with two additional arguments, an interpersonal one and an intrapersonal one, for the claim that a modern day Kantian ethics should affirm a duty to promote one's own happiness.