Sharing heterogeneous data among distributed environments in a user-centric way represents today the main challenge for personalization. In recent years several techniques have been proposed to support user modeling for multi-application (cross-system) personalization, following two main approaches. A first group of researchers base their solutions on the use of a top-down approach, involving standard ontologies or unified user models (standardizationbased user modeling); in a second research direction, a bottom-up approach based on mappings between different user model representations is the envisaged solution (mediation-based user modeling). Aim of this paper is to discuss general problems connected to user modeling in order to introduce G-Profile: our multi-application user modeling system.