This chapter introduces and defines synthetic media, a class that poses rapidly increasing risk to international security. It explains in general terms the connection of synthetic media to some existing technologies, particularly paradigms in artificial intelligence. The chapter outlines the initial difficulties of regulating synthetic media technologies, while introducing the reader to the fundamental problems that synthetic media poses to international security. Synthetic media also includes a range of ordinary communications technologies that are designed and utilized for everyday civilian purposes, including augmented reality, virtual reality, computational photography, photo editing, and voice synthesis technologies. It is unstable and unreliable for regulatory purposes, in part because it infers that deepfakes are delineable and distinct from other synthetic media in the technologies and resources used to create them.