An invalid promise is one whose breach does not wrong the promisee. I describe two different accounts of why duress and deception invalidate promises. According to the fault account duress and deception invalidate a promise just when it was wrong for the promisee to induce the promisor to promise in that way. According to the injury account, duress and deception invalidate a promise just when by inducing the promise in that way the promisee wrongs the promisor. I demonstrate that the injury account is superior. I then argue that in this respect promising is like any exercise of a normative power. I conclude by distinguishing two theories of promissory obligation, a widely held view which I call the information interest theory and an alternative which I call the authority interest theory. I argue that the points established earlier support the authority interest theory over its rival.