This is a short book about the most prominent sign of our times. The simple # sign is now used so widely that it is easy to overlook the fundamental effects it has had in the structuring of public debate. With its help, statements are bundled together and discourse is organized and amplified around common buzzwords. This method enables us to navigate more easily the huge volume of online utterances, but it also increases the risk of leveling statements and extinguishing difference, as exemplified by the #MeToo debate. Andreas Bernard traces the young and spectacular career of the humble hashtag. He follows the history of the # sign, documenting its use by Twitter and Instagram, and then examines the most prominent contemporary domains of the sign in socio-political activism and in marketing - two apparently very different fields which are united in their passion for the hashtag. Theory of the Hashtag shines a bright light on a small but pervasive feature of our contemporary digital culture and shows how it is surreptitiously shaping the public sphere.