This book constitutes the refereed proceedings of the 5th Annual International Conference on Object-Oriented and Internet-Based Technologies, Concepts and Applications for a Networked World, Net. Object Days 2004, held in Erfurt, Germany, in September 2004. The 15 revised full papers presented together with an invited paper were carefully reviewed and selected from inclusion in the book. The papers are organised in topical sections on languages and models, agents and the semantic Web, supporting software processes, software product lines, and case studies and visions.