This book constitutes the thoroughly refereed proceedings of the 21st International Conference on User Modeling, Adaption, and Personalization, held in Rome, Italy, in June 2013. The 21 long and 7 short papers of the research paper track were carefully reviewed and selected from numerous submissions. The papers cover the following topics: recommender systems, student modeling, social media and teams, human cognition, personality, privacy, web curation and user profiles, travel and mobile applications, and systems for elderly and disabled individuals.