Why do some ideas spread widely, while others fail to catch on? We review the psychology of information spread, or the psychology of “virality.” Similar types of information tend to spread in many contexts, both online and offline. This is likely because similar psychological processes drive information spread across contexts. We explain how these psychological processes interact with structural features of information environments, including norms, networks, and incentive structures. Surprisingly, widely shared content is often not widely liked—a phenomenon called “the paradox of virality.” We discuss the strengths and limitations of the virality metaphor. We also discuss future directions for the field, such as leveraging recent advances in artificial intelligence to better understand how information spreads across cultures and contexts.