What are conspiracy theories? And what, if anything, is epistemically wrong with them? This chapter offers an account on which conspiracy theories are a unique way of holding a belief in a conspiracy. Specifically, conspiracy theories are taken to be self-insulating beliefs in conspiracies. On this view, conspiracy theorists have their conspiratorial beliefs in a way that is immune to revision by counter-evidence. It is argued that conspiracy theories are always irrational. Although conspiracy theories involve an expectation to encounter some seemingly disconfirming evidence (allegedly planted by the conspirators), resistance to all counter-evidence cannot be justified on these grounds.