Virtual agents are back, and they're everywhere! Their user interfaces tend to be simply those of instant messaging... or none at all. Thus the user experience resides more in the sequencing of bits of natural language than in that of menus or screens. Although everyone knows how to engage in human conversation, creating an app that behaves like one requires a technical knowledge of the mechanics of human conversation. While Conversational UX Design is still a nascent discipline, formal models from Conversation Analysis offer a scientific foundation for design. This session from SXSW 2017 provided design principles and models for creating conversational UX.