In this chapter we present a model for the design of a conversation space to support knowledge-building. While we focus on online environments, the model has much greater generality. The model, an expansion and adaptation of Nonaka’s work, considers knowledge as consisting of complementary explicit and tacit dimensions. It argues that these two dimensions of knowledge are mutually reinforcing, inseparable and irreducible and thus in order to build robust knowledge we must attend to both dimensions and, most critically, the relationship between them. Our model conceptualizes the development of knowledge as a spiral between the complementary processes of Externalization (through collective online reflection) and Internalization (through conscientious local practice) and discusses eight principles for designing online conversations to foster effective Externalization, thus promoting the knowledge-building spiral. The broader message of this chapter is that designers need to expand their frame for thinking about “online” learning to include not only the virtual space but also the local spaces which learners inhabit in order to create useful and engaging learning experiences. All of the eight design principles presented here support this consideration.