Many extremist ideologies rely heavily on conspiracy theories to explain how the world works and where power lies. This chapter explores what our understanding of conspiracy theories – where they come from, how they work, who believes in them – can tell us about dealing with extremism. The US presidential election campaign showed that conspiracy theories are increasingly becoming part of the currency of democratic politics. Is democracy itself becoming more extremist? Where do the boundaries lie between the contestation of democratic values and the repudiation of them? This chapter examines the relationship between harmless conspiracy theories, dangerous extremism, and the rise of ‘post-truth’ politics and asks how we can still draw the line between them.