Law and Philosophy. forthcomingWe hope that criminal punishment will persuade offenders to morally reform themselves. For instance, the experience of imprisonment ought to guide offenders to reflect on their wrongdoing and recognise the need for change. This ideal of moral persuasion is frequently invoked by philosophers of punishment, but it remains unclear how punishment might communicate this moral message. Until we answer this, we will not know whether our current practices of punishment are compatible with this reformatory ideal. This is particularly pressing in the case of imprisonment, given the significant moral and material harms it produces. In this article, I draw on communicative theories of punishment to examine how punishment might morally persuade, and evaluate imprisonment by this standard. I argue that prisons are structurally ineffective at morally persuading offenders and risk undermining any such process. Considerations of moral persuasion therefore provide a pro tanto case against imprisonment.