Fake news and other forms of viral misinformation undermine our ability to address pressing crises from climate change to infectious disease. Misinformation presents new challenges for policymakers, public health officials, scientists, journalists, and others responsible for addressing these crises and communicating facts to skeptical publics. In this Master Class, we will discuss recent cases of political and health misinformation around the world, why misinformation is often so difficult to refute, and the efficacy of different approaches to correcting misinformation