This book includes peer-reviewed articles from the 12th International Workshop on Spoken Dialogue System Technology, IWSDS 2021, Singapore. Nowadays, dialogue systems or conversational agents have become one of the most important mechanisms for human-computer or human-robot interaction that has been widely adopted as new paradigm for many applications, companies, and final users. On the other hand, recent advances in natural language processing, understanding and generation, as well as a continuous increasing computational power and large number of resources and data, have brought important and consistent improvements to the capabilities of dialogue systems enabling users to have more productive and enjoyable interactions. However, on the threshold of a new decade, the current state of the art shows important areas where improvements are needed such as incorporation of ground-based knowledge, personality, emotions, and adaptability, as well as automatic mechanisms for objective, robustand fast evaluations, especially in the context of developing social and e-health applications. In this 12th edition of the International Workshop on Spoken Dialogue Systems (IWSDS), "Conversational AI for natural human-centric interaction" compiles and presents a synopsis on current global research efforts to push forward the state of the art in dialogue technologies, including advances to the classical problems of dialogue management, language generation and understanding, personalisation and generation, spokena and multimodal interaction, dialogue evaluation, dialogue modelling and applications, as well as topics related to chatbots and conversational agent technologies.