The aim of this workshop is twofold. First, it aims to grow critical mass in Conversational User Interfaces (CUI) research by mapping the grand challenges in designing and researching these interactions. Second, this workshop is intended to further build the CUI community with these challenges in mind, whilst also growing CUI research presence at CHI. In particular, the workshop will survey and map topics such as: interaction design for text and voice-based CUI; the interplay between engineering efforts such as in Natural language Processing (NLP) and the design of CUI; practical CUI applications (e.g. human-robot interaction, public spaces, hands-free and wearables); and social, contextual, and cultural aspects of CUI design (e.g. ethics, privacy, trust, information exploration, persuasion, well-being, or decision-making, marginalized users). By drawing from the diverse interdisciplinary expertise that defines CHI, we are proposing this workshop as a platform on which to build a community that is better equipped to tackle an emerging field that is rapidly-evolving, yet is under-studied - - especially as the commercial advances seem to outpace the scholarly research in this space.