What does it mean for an interface to be interactive and mediating? What would "interface' mean outside of technology? Where does the interface lie in the human system? Can the interface be located in tacit knowing? These are the questions this chapter addresses. It revisits the concepts of the last 30 years in relation to concepts that guide innovations now, exploring the landscape of ideas that have come to shape the concepts of "interface' and "mediation' and concepts of how we relate to each other and share knowledge.