Part I: Introduction.- Motivation.- State-of-the-Art and Current Practice in Power System Testing.- Overview of the ERIGrid System-Level Validation Approach.- Part II: Validation Methods, Concepts, and Tools.- Holistic System Integration and Testing Procedure.- Simulation-Based Assessment Methods.- Laboratory-Based Assessment Methods.- Laboratory Coupling Approach.- Part III: Test Cases, Case Studies, and Validation Examples.- Selected Test Cases.- Selected Case Studies and Validation Examples.- Experiences and Lessons Learned from the Holistic Validation Approach.- Part IV: Educational Aspects.- Training Needs.- Education Concepts and Material.- Part V: Outlook and Conclusions.- Outlook.- Conclusions