This book constitutes the proceedings of the 15th International Conference on Intelligent Virtual Agents, IVA 2015, held in Delft, The Netherlands, in August 2015. The 11 full papers, 22 short papers, and 21 demo and poster papers accepted were carefully reviewed and selected from 70 submissions. Constructing and studying intelligent virtual agents requires knowledge, theories, methods, and tools from a wide range of fields such as computer science, psychology, cognitive sciences, communication, linguistics, interactive media, human-computer interaction, and artificial intelligence. The papers are organized in topical sections such as adaptive dialogue and user modeling; cognitive, affective and social models; nonverbal behavior and gestures; pedagogical agents in health and training; tools and frameworks; turn-taking; virtual agent perception studies.