Harmonization of geological data across geological, topographical, but especially across national borders is one of the most important work steps to create a base for trans-European assessments of resource potentials and possible conflicts of use of European subsurface. In the last decades a variety of different thematic maps were developed, but often not on a similar and consistent data base. Differences in the geological & geophysical interpretation (e.g. stratigraphy, velocity-model, structural interpretation, different methods of assessments) across the borders remain unchanged and were masked by generalizations in an overview scale. In the last years these “border-discontinuities” have become obvious by a variety of 3D-modeling projects. But workflows for harmonization of different geological 3D models are yet not established and proofed. In the proposed project we will show on the example of cross-border pilot areas how harmonization across the borders can be established and maintained with the progress of the national models. To support this harmonization, three work packages which focuses on method development aspects in regard of uncertainties of geological 3D models and their visualization, cross-border modeling of fault data, and 3D model consistency accompany the work in the pilot areas. The latter aspect will be demonstrated on the basis of case studies to show how 3D model consistency can be proofed. The methodologic advantages (agreements on best practices, optimized workflows, etc.) and the gain in experience on cross-border 3D harmonization work will be a keystone for further transnational harmonization projects.