Scholars and educators have puzzled for decades about how to provide K-12 teachers with the resources necessary to support and improve teaching. New information and communication technologies have opened up infinitely more possibilities, setting the stage for a renewed discussion about what teaching entails and how best to support it. In this conceptual article, we propose a theoretical framework to understand how on-line resources can support K-12 teaching, and apply this framework to three on-line platforms. Our analysis highlights several design tradeoffs that reflect deeper tensions surrounding teaching and the teaching profession, and that direct our attention to (1) teacher agency, (2) the required expertise and effort, (3) teacher community, (4) and the desired degree of oversight. These tradeoffs imply that designers should go beyond attending to technological tools, and engage also with fundamental questions concerning the nature of teaching and the teaching profession.