Propaganda was first identified as a public crisis following World War I, as citizens discovered that their own governments had subjected them to deception and emotional manipulation. Today, it seems no less disturbing. Accusations swirl decrying fake news, spin, active measures, and, again, propaganda. But with nearly every accusation there is also a denial and, more important, a counteraccusation: that propaganda is merely a label applied to messages one dislikes, a slippery word that says more about the accuser’s politics than it does about supposed defects in communication. The slipperiness surrounding propaganda has fascinated scholars for over a century, as they have grappled with whether and how it can be distinguished from other kinds of rhetoric. One crucial sticking point concerns propaganda’s means of persuasion. It is commonly supposed that propaganda relies on falsity, emotion, and irrational appeals. However, adjudicating what is true and reasonable is not as clear-cut as it may seem, and much work attempts to differentiate manipulation from legitimate persuasion. Another key concern is the morality of propaganda. Some theorize that it is intrinsically wrong because it seeks its own partisan agenda. But others argue that partisanship is characteristic of all advocacy, and they wonder whether propaganda can and should be employed for worthy democratic purposes. Finally, scholars propose different models for how propaganda works. One model features a propagandist who deliberately targets a passive audience and attempts to move them for selfish ends. But other models see propaganda as a more collective activity, something that audiences pass around to each other, either purposefully or without any design. Difficult as it is to define propaganda, however, scholars do agree on two things: It is enormously powerful, and it shows no signs of slowing down.