This paper describes the work performed for the ARCHEOGUIDE (Augmented Reality-based Cultural Heritage On-site GUIDE) project. ARCHEOGUIDE is funded by the EU IST framework (IST-1999-11306), and pursued by a consortium of European organizations. It delivers an interactive, personalized, augmented reality (AR) guide for the visualization of ruined archaeological sites. It is based on mobile real-time computing, networking, and 3D visualization. Its users are presented with monument reconstructions, navigation aid and historical information in a user-friendly way through the use of multi-modal interaction techniques. The key features of the system are a novel hybrid position and orientation tracking technique used in determining the user?s viewpoint, navigation aid, outdoor operation, and real-time 3D visualization of monuments, artifacts, and life. This paper presents examples of the system?s use at the archaeological site of Ancient Olympia in Greece.