The Handbook on Socially Interactive Agents provides a comprehensive overview of the research fields of Embodied Conversational Agents;Intelligent Virtual Agents;and Social Robotics. Socially Interactive Agents (SIAs);whether virtually or physically embodied;are autonomous agents that are able to perceive an environment including people or other agents;reason;decide how to interact;and express attitudes such as emotions;engagement;or empathy. They are capable of interacting with people and one another in a socially intelligent manner using multimodal communicative behaviors;with the goal to support humans in various domains.Written by international experts in their respective fields;the book summarizes research in the many important research communities pertinent for SIAs;while discussing current challenges and future directions. The handbook provides easy access to modeling and studying SIAs for researchers and students;and aims at further bridging the gap between the research communities involved.In two volumes;the book clearly structures the vast body of research. The first volume starts by introducing what is involved in SIAs research;in particular research methodologies and ethical implications of developing SIAs. It further examines research on appearance and behavior;focusing on multimodality. Finally;social cognition for SIAs is investigated using different theoretical models and phenomena such as theory of mind or pro-sociality. The second volume starts with perspectives on interaction;examined from different angles such as interaction in social space;group interaction;or long-term interaction. It also includes an extensive overview summarizing research and systems of human-agent platforms and of some of the major application areas of SIAs such as education;aging support;autism;and games.