This book constitutes the refereed proceedings of the 9th International Conference on User Modeling, UM 2003, held in Johnstown, PA, USA in June 2003. The 20 revised full papers and 28 revised poster papers presented together with 12 abstracts were carefully reviewed and selected from 106 submissions. The papers are organized in topical sections on adaptive hypermedia, adaptive Web, natural language and dialogue, plan recognition, evaluation, emerging issues of user modeling, group modeling and cooperation, applications, student modeling, learning environments - natural language and paedagogy, and mobile and ubiquitous computing.