The educational impact of Generative AI (GenAI) technologies, such as ChatGPT, has received significant attention. We use the TPACK framework to discuss the types of knowledge teachers require to effectively use GenAI tools. We highlight the qualities of GenAI that make it like other digital technologies (they are protean, opaque, and unstable) as well as qualities that make it revolutionary (namely, they are generative and social). We describe how these traits affect specific knowledge domains (TK, TPK, TCK, XK, and TPACK) and explore implications for educators. Finally, we argue for a more expansive description of Contextual Knowledge (XK), going beyond the immediate context to include considerations of how GenAI will change individuals, society and, through that, the broader educational context.