The internet brings new urgency to the study of folklore. The digital networks we use every day amplify the capacity of legends to spread swiftly, define threats, and inform action. Using the case of a particularly popular digital bogeyman known as the Slender Man, Andrew Peck brings the study of legends into the twenty-first century. Peck explains not only how legends circulate in the digital swirl of the internet but also how the internet affects how legends seep into our offline lives and into the mass media we consume. What happens, he asks, when legends go online? How does the internet enable the creation of new legends? How do these ideas go viral? How do tradition and technology interact to construct collaborative beliefs?  Peck argues that the story of the Slender Man is really a story about the changing nature of belief in the age of the internet. Widely adopted digital technologies, from smartphones to social media, offer vast potential for extending traditional and expressive social behaviors in new ways. As such, understanding the online landscape of contemporary folklore is crucial for grasping the formation and circulation of belief in the digital age. Ultimately, Peck argues that advancing our comprehension of legends online can help us better understand how similar belief genres—like fake news, conspiracy theories, hoaxes, rumors, meme culture, and anti-expert movements—are enabled by digital media.